For me the decision is a lot more easy then for other multi console owners.
My 360 is currently 2 years old and sounds like a helicopter taking off when I insert a disc. I think it might be on it's last legs. So I'll probably be sticking to purchasing future multiplatform games for my PS3.
Things that could change my decision though:
Reviews: If one version scored substantially better than the other.
Controller: I prefer the Dualshock for Sports, Racing and Fighting games. The 360 controller for Shooting games.
Acheivements/Trophies mean absolutely nothing to me.