They both look really good. Just different art style. I think in terms of facial detail, Heavy Rain wins, hands down. For facial animation, both pretty close, with Heavy Rain maybe having a slight edge. Character animation is probably a dead tie. But, interaction with objects goes to UC2. I think actual body models would have to go to UC2. Heavy rain has a lower poly count, especially in their hands. LOL, that's probably one of the things that bugged me the most was their nasty looking hands. And finally, backgrounds are probably a tie, but you have to give it to UC2 because they are so much bigger, with much more in them.
So basically a tie. I still would go with UC2, but Heavy Rain is right on its heels. But as someone already stated, GoW3 takes the crown.