Keeping it in the water all the time is pretty hard on the dinghy. Why not put it on the dock (with a rack that holds it vertical) when you don't have it in use? Otherwise you'll probably be hauled it out every couple of months to clean it anyway. If you aren't using it regularly (weekly?) the dinghy will last much much longer if you keep it deflated and put away when not in use. I bet a PVC dinghy will last only a few years if kept out in the sun, but go for decades if mostly kept rolled up and only inflated when it use. It takes me about 5 minutes to unpack and inflate my 7'7" Zodiac Zoom.
I keep my dinghy in the water all season long early May 'till end of October. we botom paint it with inflatable rubberized bottom paint. We never haul it out to clean it during the season. We're in the Northeast so we don't have the sun that you have to deal with in the southern climates. I had a West Marine (Zodiac) PVC dinghy with a plywood floor for ten years. I just replaced it this winter. I go to my boat usually at least a couple times a week, there is no way I want to be hauling out my dinghy everytime I go to my boat. If I didn't use it as frequently then maybe. Everybody's situation is different. You really can't generalize.
My new dinghy is a PVC. If I can get 10 yrs out of a PVC dinghy with the hard use I give it, I'm happy to replace it every 10 yrs with a purchase price of about half of what a hypalon costs.
If you're considering two sizes, I would go with the larger. As was mentioned, if the if there is any wave action there's a good chance you'll catch a lot of spray, so the bigger the better. Also, while most of the time there might only be two people how often do you have load more people and supplies? If you have access to a dock where you can load up, then it's not a problem. If you get a hard floor (plywood or aluminum, breaking it down and setting it up will take more than a few minutes. However, I like having a hard floor, (old one was plywood, new is aluminum) because you don't have to worry about leaks in the air floor. The advantage of an air floor is that it's lighter and probably easier to set up and breakdown.