No offense, but comparing having your feet down from a pontoon to the way your entire lower half is down from a float tube is like comparing apples to oranges. A float tube can and will get turned over if you run aground in shallower water, whereas a pontoon is stable enough to ride it out, and you CAN tuck your legs up when you get into trouble.
I'd never, ever, ever, ever float the bow in a tube... or any river for that matter. one mistake and your done. Once it is upside down it's pretty much game over.