I've used both fives and tens on similar rafts. (Indeed, I have an 18 HP Tohatsu four stroke that burns a lot of fuel to yield only about twice as much speed as my Honda five.) A ten horse is going to push you a little faster, but not all that much. After all, you're not trying to get up on plane here -- you just don't want to have to row out against the wind, right?
Some of the performance you get will be related to prop type -- a five with the right prop will be more efficient than a ten with the wrong one for your hull type. There are charts and formulas available to figure that out. Also note that a ten horse four stroke is going to weigh at least 50% more than a five horse, if that makes any difference. My older Honda five weighs 65 pounds, and the new Honda 9.9 long shafts are listed at 92.
Motor type is a consideration if you're thinking about doing rivers or impoundments that currently prohibit two-strokes -- as in Grand Canyon. And thinking ahead, it seems likely that more and more river management agencies are going to require four strokes. So looking to the future, buying a four stroke now may just save you from having to go out and hunt for one, while trying to dump the old two stroke.