the 360 version may indeed not sell more, but the possibility of one will be more than enough to hinder any success the PS3 version could achieve.
On another thought...
Even an acclaimed game like Bioshock was limited in its sales by its approach towards the FPS genre, it simply didn't appeal to as many as a game like Halo did. No matter how good MGS4 is don't think a franchise which barely sold a million with it last installment (MGS3) can break records with the fourth.
I have a feeling we'll be somewhat dissapointed with MGS4's reception.







