I'll be getting MW3 tonight at midnight...but i prefer the Battlefield series.
Why?
I prefer team/squad based play...modern warfare is just like Halo, run-shoot-run-shoot-run-shoot-camp-shoot-camp-run-shoot. That's it, all day and all night. Battlefield actually makes you work as a team (in Rush and Conquest). Nobody plays the other modes on CoD, its Deathmatch or nothing pretty much 100% of the time.
The campaign is ALWAYS better with the Battlefield series, but i still enjoy the CoD series campaigns because i love shooters.
Fit and finish was better on Bad Company 2 over Black Ops, and the battlefield game was almost a year older.
Give me Battlefield any day of the week...if i had to pick one, that would be it without a second thought.
The only thing that ever drew me to keep CoD games was the Zombie modes, which is very fun.