The Duracell GC-2's are miles ahead of any 12v marine dual purpose battery in a deep cycle application. Much more tolerant of chronic undercharging, much easier to recharge and in general much longer lived.
The only advantage of 12v batteries, is of one can fit 3 batteries, or is powering high amp draw devices like microwaves. Golf cart batteries have low CCa ratings, as they are not designed for starting duties. High CCA ratings are good for powering huge loads. they will hold higher voltage under those loads.
But from a Deep cycling perspective, two Flooded batteries of the same size and voltage, the one with a lower CCA rating, will be the better deep cycle battery, as it has thicker denser more durable plates. Thinner porous plates are good for quick bursts, but are fragile and prone to sulfation, and the process of desulfating via equalization or high amp recharge rates can cause the starter battery plates to shed material.
The Duracell GC-2's are No trojan T-105, but they will blow away ANY 12v flooded battery, all other factors being equal, in a deep cycle application.
That price is good. Look for the date codes, and see if a manufacturer, like East Penn/Deka or Johnson controls is listed.
I'd choose East penn over JC if given a choice. I'd avoid Exide. Crown and USbattery also make top notch Golf cart 6v batteries.
Crown, trojan, USbattery and East Penn all seek to get all golf course business and put huge efforts in increasing cycle life.
Trojan ans USbattery practically goto war over this, So while lead acid batteries is ancient technology, it is not fully mature. They are tweaking plate paste formulations just to get an edge over each other, and their formulations and processes are closely guarded secrets.
They don't put 1/5th the effort into 12v batteries.
Anybody who can fit the taller 2 GC batteries in series but instead chooses 2 12v marine batteries in parallel, is making an unwise decision, 99% of the time.
Most of the 12v batteries which are marketed as a true deep cycle, which are few and far between and much more expensive, still can't compare to a 6v golf cart battery in durability.
A pair of GC batteries in series has about 950 to 1200 CCA which is more than enough for all but large diesel engines in cold weather climates. deep cycle batteries are not damaged by starting engines, as some people like to believe. it is just that they were not designed for providing large short bursts of energy, and are not ideal for that purpose, especially when weight is a consideration
