MW3 in my opinion will be a refurbished version of its predecessor. BF3 has added combat dynamics with a new game engine allowing for even greater real-time combat options. I'm not saying that MW3 will be exactly like its previous game, but there will be strictly too many similarities. Of course if you are a fan of the COD series, then it's what you'd most likely stick to.
However, if you've never played either, I think BF3 has a lot more features that crush MW3.
There are plenty of game reviews comparing both games and I believe more than 50% favour BF3. If you don't believe me, a Google search is only a click or two away.