The differences I see is the 360 version appears to be brighter, with stronger colours, while the PS3 colours look a little more mutued. Up close, the 360 version also seems to have slightly more detail on characters faces, wrinkles are visible in some shots while they look absent on the PS3 version.
The PS3 version seems the have the better draw distance, some buildings in the distance are not visible in the 360 version, that said however in one shot it looks as though the 360 version is showing a mountain in the distance, while on the PS3 it looks like a building is popping up, not sure what's going on there. It also looks as though the PS3 has better AA effects throughout.
Minor differences really, nothing gamebreaking. I they have the same framerate, then everything is basically on a par. If there is indeed multiple discs for the 360 version I would choose the single disc over that.
