Obiously both the games are great, but i would pick modern warfare 3 because i really can‘t stand big maps like the ones in BTFL, i prefer close quarter maps where you have to rush and kill everybody on your way. To be honest i bought Battlefield 3 but 4 days after i would not play it just because i am not good at it, the CD is still there probably covered with some dust lol.
I think it's silly to compare them, the gameplay is just so different in both.
That being said, I enjoyed both. Though, I only played BF3 for a few months when I was still living my roommates then. When he moved out he took BF3 with him, and since I haven't been motivated to buy it. Not because it's bad, just because of money problems. XD
Would I buy BF3 for myself? Maybe. But while we're on the topic, can anyone tell me if the DLC for BF3 is required to play online nowadays, or can I play the vanilla game only?