I'm getting in on this a little late, but here is my $.02. I have both a float tube and a pontoon, similar to this. I use the tube when I am going to be on still water for a short time, and I am not too far from shore. The 'toon is much better on rivers (even down riffles), moves faster across larger still waters, and can still be maneuvered with fins to keep your position or troll. For the extra $$ it will provide many more options, and in fact will be safer on even a slow river because your feet won't be caught on any shallow water, causing you to tip.
Plus you can attach a pack to the back of the chair or (in some cases) on the small platform behind the chair there is space for a small cooler – so you can stay out longer, or have a snack on the far shore. I agree with others, get a pontoon.