This might be illuminating for some of us. N4G has an article (see below, and DON'T CLICK on it unless you don't mind seeing some of the in-game content -- stills only) that talks about the two ports. Interestingly, they say that the game has achieved "near parity" on both consoles, with the Xbox version having shadows and vegetation that look slightly better. They also wrote that framerates are equally good on both systems. N4G emphasize that the Xbox version is only "barely" better-looking, and that they have no idea yet about which version will perform better on-line.
Read it for yourself (again, DON'T CLICK if you want to avoid seeing anything from this game yet):