Well, I had good times in both of the game series. My favorite CoD game is MW2, got a alot of hours in it. I loved playing it, alone, in a group or in a clan. It's just sad that 90% of your enemies are not older than 12. Additional to that in every third lobby there's a hacker.
My favorite Battlefield game is Battlefield 3. Got it before it got released, and I'm still playing it. I could give you a load of reasons why, but I'm not going to bother you. Additional to that there are nearly no hackers, and even if there's one, he gets banned pretty quick.
After all I prefer Battlefield for it's atmosphere, it's feeling and the handling the game has.