This is a tough one, with financial obligations, I had to chose one to reserve and make steady payments plus trade in finished games to pay it off. It was definately difficult for me. Loved Infinity Ward's take on Call of Duty, their Modern Warfare story was supperior to Treyarch's takes (which cant seem to leave the past). It's story left me wanting to see what's next. Black OPS wasnt that good a game for me. I never really cared for the zombie mode like other players do.
Then theirs Battlefield 3, watched clips on it and damn, the graphic quality is outstanding. Never played Battlefield 1 and 2, but only the offshoot, Bad Company 1 and 2. I was impressed with the graphic quality of both games, though I liked the story better in the first game, yet they deviated from in the second one.
But after brainstorming a bit, I stuck with Modern Warfare 3 because for me its a proven brand.