Not interested in either.
My first priority in what I look for in a game is a good single player experience. While the original Modern Warfare had a great story, it's not something I'd want to pay 60 bucks for. And since then, the CoD campaigns have gotten shorter and more rediculous so I'm even less willing to pay money to access the experience.
When it comes to multiplayer, I'm more interested in co-op than competitive. The spec ops missions the new survival mode in MW3 look pretty fun, but, again, that's not enough for me to be willing to pay 60 bucks.
Competitive online multiplayer is something I can play for 2-3 hours, think to myself "that was pretty fun," then move on. I may play it more if I have real life friends to play with, but none of my friends play CoD or Battlefield, so that's out. In the end, the head-to-head multiplayer just doesn't have lasting appeal for me.
If I was given a copy of MW3 or BF3 through no personal expenditure, I'd play it, and I'd have some fun with it, but it's not the kind of experience I'm willing to pay my own money for.