If I had to choose? Battlefield 3. Its Multiplayer is just fantastic.
But really, CoD is just as good in different ways. BF3's campaign was pretty forgettable, especially considering I couldn't wait to play the epic multiplayer, but it still had some memorable moments. The dogfight above Iran and the Uprising mission are prime examples. But MW3's campaign was just... epic. Sure, it was a bit weaker than MW1 and 2's campaigns, but what are you going to do?
MW3's multiplayer was, lets face it, flawed and repetitive. Battlefield wins here in every way. Same thing with graphics. You just can stand up to Frostbite 2. But, like I said, MW3 is still good in its own little ways.
I'm looking forward to playing BO:2 and seeing if they improved and gave it some more variety.