Firstly i'm going to talk about spawn killing, the most annoying of all kills. Before you even get to fire one bullet PLOW you're blown to bits. This has only ever happened to me on C.O.D games on Battlefield it's NEVER happened to me.
The graphics of battlefield may be arguebley less smooth and detailed (definitely not Black Ops though), but who cares we have HUGE maps, and they certainly look more realistic - i think that's much more important when it comes to a good fps. I mean seriously, look at this image, a great big map, a great big gun and a realistic looking environment, what more do you want?
Oh wait, a bit of balance maybe; there's no point in even picking up a sniper rifle in Black Ops, and pretty much all guns are just ridiculously unbalanced. Whereas in Battlefield its well balanced and every gun and class has its good and bad points.
One of the most important things about a first person shooter is that when you shoot someone in the face point blank with a shotgun you expect them to die. Not always in C.O.D. Battlefield however saves you from any of that hit detected frustration and your left with a satisfying feeling of just murdering a man a split second before he was about to murder you... ahh virtual violence.
Finally the maps. I've already spoken about how huge Battlefield maps are and how well placed the spawn points are, but that isn't the cheery on the cake. In battlefield you can frekin blow up buildings, knock through walls with explosives. Got an annoying camper hidden in a building? BLOW. THEM. THE. HELL. UP. If it was C.O.D you'd be running in and dying 10 times in a row before you hear a hoard of angry 12 years olds screaming 'camper! camper!' down the microphone shortly before they cry themselves to sleep at night over a poorly made video game they should have never bought.
With Battlefield 3 well on the way too i'll be surprised if C.O.D survive's much longer.
I think the choice is pretty obvious myself!