Most likely both will be fun.
MW3 will be more over-the-top than ever, and BF3 will also show a fancy campaign. Of course, MP is all the fuzz these days. MW3 will be fast solo action, running around, gunning, camping, whatever. BF3 most likely will need quite a lot of teamwork, but there's always room for campers and whatnot.
Anyways, I'm pretty sure I'm gonna enjoy both of them, they have stuff that i like.
Also, i don't think versus threads are good forum material, i think somewhere in the forum rules there is a link between the words "versus threads" and "frown upon". I think. Not sure.