on my mtb i always run a WAAAY bigger front tire for increased cornering and bump absroption. no worries at all.

i'd always stick the bigger one on the front. also, the rear tire seems to contribute more to rolling resistance than front tires so having a higher psi smoother tire back there will get you the most performance advantage.

plus as the other poster said, ideally you want your rear tire to wash out before the front. not that you really want either washing out but i hope you get the idea