Well, both games have their ups and downs.
MW2: More focused on a single person than teamwork. Sometimes has lag issues, and bullet hit detection sometimes fails.
BC2: More focused on a teamwork view, it's like real war. Rarely lags, and no bullcrap kills.
I, personally, like MW2 solely for Search and Destroy, however, BC2, is a great all around game, especially if you have a team to play with.