You also have to bear in mind COD's competition on each platform.
On Sony you have Killzone which sells okish and Resistance which is non existant sales wise.
However on 360 you have Halo which is a 10+million seller and as a result 360 owners are obviously torn between the 2.
IF Halo was selling 2-3 millon like Killzone, then you would probably find these FPS fans buying the likes of COD instead.
I think things need to be put into perspective in the OP. Because different things affect each platform.Whether you agree or not with Halo's quality, fact is it gives COD a much harder time for sales on the 360 than any FPS on PS3 callenges COD.