So far, the majority of votes are either BF3...OR BF3 for MP but MW3 for SP...
I have agreed that MW3 will have a pretty good storyline to continue it's sequel on MW2, but I will probably watch every single of its campaign on Youtube (like I did to Portal, Portal 2, HL, HL2, HL2 EP: 1,2, and next will be 3).
The feeling you get watching it all online is equivalent to just playing it by yourself...plus MW3 will probably cost a lot like the rest of its sequel and if I had like 50-70 bucks and had to choose between MW3 and BF3, I'd choose BF3 for its *NEW* Campaign + Multiplayer.
MW3's multiplayer is kinda boring and gets you bored fairly fast...especially the lack of hack protection just like MW2...
BF3 will probably make you stick to the game for YEARS...and with 2 years of playing BF2, I've only saw ONE hacker and nobody cried about it either.
But like I said, MW3's storyline will probably be awesome...but we never seen a storyline in BF2 before...so BF3 will probably be another good storyline.