Skyrim
I've come to this dilema a few times and decided: Skyrim.
If I were to choose between the two, however, I would have to go wih MW3. Yes, Battlefield is "Bla bla game of the year" named by some person no one has ever even heard of, but it is a team-relying game. If your team isn't decent (greifers, non tryers or just plain people who are bad), you will not win anything. Maybe it is the fact that I find the Battlefield series horrible after Battlefield 2, but nothing screams "Waste your money on me" about it. I did buy BC2 when it was $5, and I am still unranked. Over all, I believe the game looks a little too over rated.
As for MW3, it isn't as big of a bang as WaW to Black Ops, but it is more appealing than BF3. It is about personal skill (Much like what CS:S has become) and has, in my opinion, a much more incentive based and funner leveling and customization system.