Get the fresher of the two batteries. They Likely weigh the same and are internally the same, the only difference being the stickers. This is a guess with no Data to back it up, only suspicions and a huge contempt for product marketing, as practiced in this day and age.
No 'true' deep cycle batteries exist in a 12v format, but Trojan, Crown, Deka and USbattery make higher quality dual purpose/marine batteries in group 24/27/31 sizes which cost about 25% more than their lesser Everstart, and similar marine Autoparts store counterparts.
Even these best offerings will only have half the rated cycles as a true deep cycle battery like a T-105 golf cart battery, and these better dual purpose batteries in the same usage will likely exceed 25% more cycles before failure that their cheapest counterparts.
There are true deep cycle 12v Jars, but they are much taller such as the the Trojan T-1275 and J-185. A group31 trojan scs225 and a t-1275 are the same price in my location, and the t-1275 has 20more AH capacity and twice the plate thickness, and 22 more Lbs of lead and is one of the very FEW 12v flooded batteries that is a true deep cycle battery
The amount of solar which can keep a battery happy is entirely dependent on the depth of discharge of the battery. The deeper the discharge of the battery, the higher the solar wattage needs to be. Meaning a 100AH battery cycled to only 80% can be perfectly happy with 100 watts of solar, but the same battery discharged to 50% will not be as happy, even if the sunlight is of long enough duration that the charger drops into float mode/voltage.
The charge rate plays a big factor in battery longevity, more so on AGM than flooded batteries. Most flooded manufacturers recommend a 10% rate, or 10 amps for 100AH of capacity and assume one has 8 hours or more to recharge. With the slow ramp up of solar and a tapering off as the sun rises and falls, meeting specific recommendations as to charge rate becomes fuzzy, and the easiest plan is, more is better.
With AGMS, some of them absolutely require HUGE recharging amps be applied from their most depleted states. Odyssey say 40 amps minimum for 100AH of capacity. Others like UB battery, say no more than 30 amps per 100AH of capacity. But even these amp limited batteries benefit from charge rates that approach their stated maximums. Again the lower the battery is discharged, the higher the amps it would like to see, initially, and light levels of depletion, then lesser charge rates are likely adequate. AGM batteries are often painted with the same broad brush, but their actual requirements as to recharging vary very widely among manufacturers. AGM batteries prices often fill the buyer with 'Super battery" status and bragging rights, but if the charging regimen is not tailored to the Specific AGM, then this super battery will give no better service, or worse service than a flooded battery that cost less than half.
Often claims are made as to a battery working just fine, or still going strong, but the level of discharge is unknown, the total amount of accumulated cycles is unknown, the remaining capacity of the battery completely unknown, and claims of 'just fine, still going strong' only mean the battery capacity has not yet depleted to the level that it compromises the depleter's ability to power their devices. Whether the amount of time and cycles accumulated are just fine, or not, is highly subjective.
Now if a 50% discharged battery were well wired to the alternator via a solenoid or other non Diode based isolation device, and the vehicle driven for 15+ minutes in the morning, when the battery is at its most depleted, then the battery has a much higher chance of being happier.
As said in previous responses, most people new to living on 12v are battery murderers, and the first battery a learner battery. Put a cheap voltmeter on it, one that you can easily see. When discharging under light loads such as LED lights, disconnect the light loads when the voltmeter drops below 12.2v. A voltmeter is not a true battery monitor, which counts amp hours into and out of the battery, but voltage is a clue and the more one observes voltage during charging, and discharging, the better idea one gets of the state of charge of their battery, and how it is performing as it ages.
This is not a specific product recommendation, just an Example:
http://www.amazon.com/Mictuning-Dig...1448227885&sr=1-18&keywords=digital+voltmeter
Beware of those voltmeters that plug into ciggy plug sockets, not only are some highly inaccurate, but the ciggy plug itself might be on a circuit that has other loads, which will throw off the voltage readings. One wants the voltmeter reading the battery terminals themselves, not tapped into another circuit halfway through. Ciggy plug voltmeters often will read low, and the user will stop discharging before they really need to. Better for the battery, but inconvenient for the user
When discharging under heavier loads such as a laptop whose internal battery is recharging, one can go lower, to the 12.0v range as the voltage will rebound to 12.2 or so with the load removed.
50% charged is around 12.2 volts, on a well rested battery, one which has not seen charging or discharging for many hours. This is unlikely to ever occur in actual use whilst dwelling, so some guestimating has to be done, and a 'close enough' call made.
The battery each day during daylight hours needs to spend at least 2 hours at absorption voltage of 14.4 to 14.8v to be happy enough. Maxing out the Specific gravity, which is the only real way of confirming a true 100% recharge, usually takes closer to 4 hours when held at absorption voltage on a healthy battery. An unhealthy chronically undercharged battery might need to be pushed to equalization voltages of 16v to max out Specific gravity. Often EQ charges are applied as a final Hurrah on a battery that is too far gone to recover enough lost capacity to be useful again, but applied somewhat regularly, they can greatly extend the lifespan of the battery.
AGM batteries are 100% charged when the current required to hold them at absorption voltage has tapered to 0.5amps per 100AH of capacity.
When A charge controller drops to float voltage, this DOES NOT MEAN the battery is fully charged, it means only that the controller held absorption voltage as long as it was programmed to do so. Many people see their charge controller in float and think their battery is fully charged, but it really means very little unless confirmed. The amount of time a battery needs to be held at absorption voltage is different for each and every battery, its level of discharge, health, temperature, and charge rate, so a 'One size fits all' algorithm, is more like a one size fits none. Good enough or NOT? impossible to say without confirmation.
Wiring up a simple continuous duty solenoid to charge house battery is no more difficult than hooking up a solar charge controller, but the copper should be thicker, at least 8awg, with 6 being better, 4 better still, ect. The distance to the house battery plays a huge part in wire size needed, and batteries can accept huge recharging currents when discharged below 80%, safely. It is the battery itself which dictates how much amperage it can accept at the voltage allowed by the vehicle's voltage regulator. The alternator only makes as much current as required to attain the voltage the Vehicles voltage regulator allows. Many people think an alternator can overfeed a battery. This can only happen if the Voltage regulator allows too high a voltage for too long, and as this is dangerous, from a lawyerly point of view, the opposite is more likely to occur, Too low a voltage held for too little time in most vehicles.
A low slow recharge, is better, only when one has 24 to 48 hours + to recharge. When the next discharge cycle begins late afternoon, then it is much better to high amp recharge a battery via an alternator, then the low and slow solar to finish it off, to give it the best chance of attaining the highest state of charge possible before that next discharge cycle begins.
Do this and even a low/ middling quality marine battery will give good enough service
As far as alternator charging goes:
http://www.expeditionportal.com/for...ke-a-cheap-isolated-dual-battery-setup-for-50
While the diagrams show that he takes power for the solenoid from the engine battery, Another option for those seeking more alternator contribution to depleted batteries is to take power for solenoid from alternator(+) stud instead, as this can make for a shorter circuit, meaning less $$$ copper, and it also bypasses the OEM alternator charging circuit wiring, which is too small for the additional load of an additional depleted battery tacked onto the end of the circuit. The other benefit is the that the vehicle's voltage regulator will be able to 'see' the depleted battery and allow higher voltages to be held for longer, but how long and how high is highly platform dependent. Voltage is electrical pressure, so a higher pressure allows more amperage to flow.
A depleted battery hooked to an alternator over thick copper can present a huge load to the alternator. The Alternator's life will be shortened to some degree due to the incredible amount of heat it can make when maxed out. Some alternators can produce huge currents at idle. Such alternators at Idle, their internal fans are not spinning fast and not moving much air through them. Combine this with not much airflow moving through the engine compartment when not moving, and the alternator 'might' quickly fry itself. I recommend NOT idling to recharge, but driving instead, unless one can confirm their alternator is not above 220F when idling. The cooler the better, but 220F is a tipping point that will quickly degrade the diodes and bearings in an alternator. An Alternator at max output or near maximum output will heat up faster than the engine spinning it.
Some other alternators might not be able to produce much current when idling when hot, and idling can do very little but burn gas. Again, highly platform specific so without the ability to see how much current is actually making it to the battery, Idling to recharge might be a foolish wasteful endeavor.
If one's alternator is very expensive, and requires lots of labor to replace, then it makes little financial sense to burn it up trying to keep a battery happy. One can limit its output by using thinner cabling to the house battery, such at 8 or 10awg, and still get a good 18 to 35 amps, where as 4 awg would allow 60 to 70 amps or more to flow initially and an overall higher amperage rate whilst the engine is running.
The alternator only makes as much current as needed to hold the system voltage at a preset level controlled by the voltage regulator. A depleted battery might require 70 amps to be held at 14.4v, but 85% charged battery might need only 9 amps. The alternator rating means very little in actual use. Alternator ratings are established in a Lab, with a cold alternator spinning very fast feeding huge loads over thick copper cabling. Ideal situation, which has little resemblance to an alternator in a vehicle in actual use.
My Alternator takes me less than 15 minutes to replace, and has a Lifetime warranty, and basically is hooked with 2awg cable, and will pass 110 amps maximum for briefly, given enough rpm and a depleted enough battery. I still got 8 years out of the alternator before the stator shorted and O'reilly's gave me a new alternator under warranty. But previous to that alternator, I exercised that warranty much more often, as I did not have Solar then, nor a good understanding of battery charging.
While I recommend having alternator recharging as an option, many people can and do use solar only to recharge, successfully, so far.
How much longer their battery might have lasted had they had the option of alternator recharging, is unknown, but if the sun goes behind clouds for days on end, the person who can recharge while driving, doing a supply run, is going to have that much more electricity to use for longer. The person who resupplies and does not take advantage of the alternator's ability to recharge enroute, is going to dream of thick copper between house battery and alternator and solenoid.
Alternator charging can always be accomplished later on, IF/when the solar alone proves to be inadequate, and when one feels more comfortable with their skills to install it themselves.
Keeping a battery happiest is almost impossible. Ideally it would be fully charged, and kept of a Float charger and never discharged at all. that is ideal from the battery's point of view. A battery that is actually discharged regularly has to be recharged properly to be happy enough.
Happy enough is a pretty wide spread and highly subjective to the human who purchased the battery and can only really be determined my most people, when the battery fails to meet their capacity requirements. Before failure it is 'just fine' and 'going strong', and like a switch was thrown, the next day the battery 'no longer takes a charge'.
It really comes down to a total accumulated cycles over a certain timespan as to whether any given recharge regimen is good enough, and the battery longevity good enough. Total cycles accumulated per dollar.
I once was going through wally world batteries every 6 months, the actual $$ spent not all that much, but it was a Pain in the Ass when the battery capacity was so little that i had to constantly worry about if I had enough juice to power what I wanted to power, and if I could actually get to a walmart which would warranty the battery.
Not being a people person, I find dealing with Wal-mart, its employees and clientele, to be stressful, and it really promoted my learning of how to keep a battery happy, so as to avoid unpleasantries which occur when they need replacement, prematurely.