There are a massive amount of 360 owners who play only a few games, those games being Halo and the Call of Duty games. Really, outside of that, software sales numbers are fairly equal for exclusives on both consoles
Also consider Ratchet: A Crack In Time, its numbers seem small, but insomniac has stated that those numbers are consider very successful. A lot of these franchises would rather the game sell continuously over time than just a huge blowout the first couple weeks. Just depends what goals you are trying to meet.







