I've read through several articles where they compare a game over the 2 systems and see which is better. Games like Ghostbusters and Crysis 2 the only difference was some of the textures and things like harder edges on shadows and the like.
At the end of the day if you are actually playing the game and not just sitting there looking at it you're not going to really notice the difference without both systems running on side by side screens.
The PS3 does have the one up though since it uses Bly-ray disks and can fit more data. If i recall Final Fantasy XIII was something like 3 dvd's on Xbox and 1 bly-ray disk on PS3.