It is minuscule, which is why I mentioned that it's not by much. But I recommend you read Eurogamers Face-offs. Nearly every single game, with the exception of maybe 3, the 360 game looked better. They even have side-by-side videos to prove it.
Now I'm not saying that the differences are enough to say that the 360 is superior and you shouldn't buy a PS3 copy. But technically, the 360 port is the better looking game because of the graphics processor. PS3 anti-aliasing actually blurs out a lot of the textures. Not to mention that most PS3 ports don't even get true 720p resolutions. They also have to take out some effects usually due to the PS3s video processor unable to handle them.
With all that said, you would have to be looking for it and/or just have a good eye. When I went to school I learned a lot about video and these issues do stick out to me unfortunately. At the moment though, it seems the only PS3 games that look better are the ones that are exclusive to the PS3.