I've used lots of different solar showers over the years. They all work about the same so I would say go by price and build quality. I will say that a 5 gallon solar shower is LOTS of water. If you have 4 people showering, that is about right. I find it hard to use more than about a gallon of water for my shower (and that includes shampooing hair).
As mentioned in another post, they can heat up very quickly and get too hot. If you have a really sunny day, only fill it up about halfway so you can add cool water to adjust the temperature. If it is a cloudy day, you can always heat water on the stove and pour it in. After a few times using it, you will get an idea of how much water you use.
My last solar shower has finally bit the dust. I don't remember the brand but I found it in Big Bend National Park. We were driving on one of the back country roads and found it in the middle of the road full of water. It had apparently fallen off the top of someone's truck. We put it on our roof and had "free" showers that night. It was a real treat in the desert!
"When in command, command." -- Admiral Nimitz
Difference between a power boater and a sailor out on the water: A power boater is going some place special, a sailor is already there.
s/v Zotz 1981 Pearson 365 Ketch Hull #375