I haven't bought either yet, but I'm probably going to do BF3 and then buy MW3 later when it price drops.
My big thing I hated with all the recent COD's is that you just get bum-rushed by people who spray and pray. Whenever I played multi-player the only mode I would play is Search and Destroy Hardcore. It forced people to be tactical since you only had one life per round and the teams were 6 on 6. Plus in hardcore you actually had to have some skill. I was really good at it and I think my record for a round was 20 kills to 1 death. But the thing is most people didn't play that mode so it was always a pain in the ass waiting to get a full enough lobby.
I wish the movements in BF weren't so sluggish, but it's a much more tactical game and the massive maps are awesome.