I reckon the two games have now gone in two different directions. MW3 is just advancing on what is already known to be a great game, it's more arcade-like. Because of that, it is run around and kill people, which is fun. That's the style of the game.
BF3 is more realistic, for the more realistic player. It's so hard to see people on the huge maps, but that's the fun part of it, it's more rewarding.
MW3 is more rewarding for score, BF3 is more rewarding for actually killing someone lol