I believe MW2 sales on the 360 alone will be higher than Final Fantasy XIII sales combined from both platforms. BUT I believe the Japanese PS3 FFXIII bundle will sell more than the American Xbox 360 MW2 bundle.
I think most people who want MW2 already have a Xbox 360 and we all know Japan has been waiting for the PS3 Slim and FFXIII before hopping aboard from the Wii or w/e.