Ray Smoove has both, so he'll have a huge bias and say MW3, he was dragged kicking and screaming to get BF3
IMO BF3 is the winner for PC, but on consoles MW3 is a no brainer. The server customization, 64 player battles, its just fuckin awesome man. Just know you can't play BF3 like MW3. It's not a run n gun game (unless you play small maps & infantry only), you have to actually be smart and think about where snipers or tanks are.