Don't think breaking chains is going to be a problem with 10 speed. The roadies have been using them for ages and the pro road guys put a lot of stress on their chains.
More problematic might be that the narrower 10-speed chains clog up with dirt easier.
Chains usually don't break because of pure power, they usually break because of force applied when shifting twists the chain and pops the sideplates. 9 (and 10) speed chains are narrower, therefore less torsionally stiff.