If you check the XBL stats, Halo 3 and the call of duty games are always fighting for the first place (Gears of Wars 2 didn't survived long agains't those two...).
So, if a game can beat Halo 3 first week sales, it is MW2.
Evan Wells (Uncharted 2): I think the differences that you see between any two games has much more to do with the developer than whether it’s on the Xbox or PS3.