Yes, you will get better footprint in the water with a 10' boat, plus you'll get alot more carrying capacity with that extra foot (roughly 600#'s with 9' vs 1000#'s with 10'). You won't have that much extra weight actually. I only think between the Osprey 9-10' is about 5-10#'s max weight difference. But the bigger boat you have, the easier you can take whitewater. Say it this way, I have run whitewater in 8' up to 18' catarafts. Your smaller boats get thrown around alot more. The 18' is a smooth ride. Pretty much compare whitewatering to running 4x4's. The smaller boats are like your Jeeps, they get bounced around alot more, but are more manueverable., but can't carry as much. Your bigger boats are like the 4wd suburbans, they are bigger, smoother ride, and carry alot more, but you need a bit more maneuvering room over a smaller boat.
Hope this helps.
"Good angling ethic is not a monopoly practiced just by fly fisherman" - Bill McMillan