If you plan on using your float tube in moving water, don't. The danger of getting your knee smashed or your feet hung up in a float tube is very real. I have seen guys with some nasty injuries from using float tubes in moving water.
Even if you just want something to fish still water there are benefits to having a toon. You sit higher off the water, you can scoot around faster and most toons have more storage room. Toons may be a bit heavier, but if that is an issue there are some easily packable toons out there too.