"V" tubes are the ultimate, then "U" tubes and lastly doughnuts.
I have had all three and the Doughnut is hard to get in and out of. They are more dangerous if they should happen to flip, plus you are sitting much lower which can make casting a little more challenging.
"U" tube is still sitting a little lower and the neoprene seat thing. Both of these can be manuevered easy enough but you are pushing a lot more water specially if there is wind.
"V" tubes, you are sitting much higher (in fact in my Freestyle H3 I am sitting enough out of the water that I can wear hippers).
They track better because thay are more like a toon with two tubes tracking side by side with an arrow to cut through. Easier to cast from.
They will all fold down to nothing except maybe the ones with the styro seat.
There is also the Pontubes which are small metal frames with 4' pontoons like the Venture Outdoors "Echo" and Denny Rickards "Super Cat".
forgot to mention that the "V" boats have MUCH MORE STORAGE!
One thing to seriously think about is Urethane bladders over PVC. More money but will last longer and not as effect by temp change.