HD 7950 would perform better, not by a lot, but better nonetheless. Get it. Best HD 7950s are Sapphire's, so if you want the very maximum out of it - get theirs.
Of course, GeForces have PhysX by default, but you can do HPX
-setups with Radeons just fine, so there's really no point to get Nvidia card just because "it has PhysX".