I opted out of the 'porta-pottie' after having had one for years and years. My van camping dates back to the mid 70's when a porta pottie was all that was available. We hadn't even thought of buckets and bags back then!
Having used several brands over the years there wasn't much difference in them. Some had slightly larger bottom tanks so that you could get an extra day of use out of them before having to find someplace to dump them. Because they need chemicals one needs to find either a toilet of some sort or a sani-dump station to empty them in to.
While some people will tell you that there is no odor from them, my experience is that you either get to smell the chemicals if it's fresh (newly sanitized and set up) or the odor of a tank that needs dumping. The only time I couldn't smell anything in the van was when I essentially encapsulated it in a 'closet', then only the closet smelled or the van when you opened the closet door.
I chose to go with a marine grade separating toilet after seeing some separating/composting toilets in use at remote off the gird lodges. I am very happy with mine.
While there are several brands on the market, I chose the C-Head, not just because the initial cost was much lower than it's competitors but also because it uses a standard 1 gallon milk/water jug for the liquid containment. The next closest unit in price uses a specialty container that, if you wanted a spare (and you definitely do) would be $40 IIRC and would have to be cleaned and stored. By using a standard water jug, I can toss it away when it gets nasty (it does) and simply use a clean one. I keep a spare on hand at all times!
I like it because there is no chemicals to be used so it's environmentally friendly, the cost of operation is low (less than a dollar a month) and with a couple of understandable and avoidable exceptions, I haven't had any problems with odors.
The two times I can think of when there was a noticeable odor where both after I had had some pretty severe digestive problems resulting in more liquid in the solids tanks than there ever should be. While on the highway there was no problem but after several miles of bumpy forestry roads the solids container was stirred up to the point where it could be smelled. The cure - empty the tank if you've had diarrhea BEFORE carrying passengers in the back of the van on bumpy gravel roads...sigh! The smell went away again as soon as the tank had a chance to sit still - my future DIL wasn't impressed however...
Does the liquid jug smell when you're dumping it, not going to lie, of course it does. Does the solids tank smell when you're emptying it, yeppers! Does the porta-pottie smell bad when you're emptying it, uhhuh! Same goes for when someone is dumping their holding tanks at a sani-dump. Which smells the most - IMO - the chemical, liquid and solid mix of a holding tank/porta-pottie wins that one, hands down!
Other hints and tips - no matter what you choose to use, do NOT put the toilet paper down the toilet. It does not dissolve in the amount of time that it's in any of the holding tanks, no matter what style you choose and it clogs the drains and/or takes up space that is better used.
