Got both...
BF3 for slower, team based gameplay
MW3 for fast arcadey style gameplay
Two very different games.
TBH if someone told me they wanted one of each of those types of game i would advise them to get MW2 over MW3! you can pic it up much cheaper now.
In other words, i agree with people saying about having 2 different types of games but a better question would be MW2 vs MW3, especially if you already have MW2