Depends what you rather like but MW3 is the worst CoD ever said by so many people and I really don't like the maps... the graphics aren't better than MW2 etc. so if you are a graphic whore and like a realistic game you should rather buy BF3... BUT I think MW3 still is more fun so you have to decide yourself I think...
It has been my favorite game for about 2 years I think...
But for now I'm playing Cod: MW3 and BF3^^ Oh and I downloaded an emulator for my mobile phone again to play Pokémon <3
I am dying to play Call of Duty: Modern Warfare 3 :D I just have so many other games to knock of my list before I can.