Well it all depends on what you like:
Mw3, have a good story line and is good to play. (After what I hear and such.)
Bf3, is a good online game and also have a okey story to play (After what U hear and such)
So, Just get them both and you will soon notic what game is the best.
(I think I will wait until they maybe get on the "black-marked" and try them. If I like them / it I will go and buy it.)