In my experience with the multiplayer for both games, I much preferred Bad Company 2's multiplayer over Call of Duty, and I will delightfully explain why.
In Call of Duty, you expect to die every single second. Every second you remain alive is just one more second closer to death, skipping the killcam, and respawning. It's incredibly fast paced, and as such the gameplay, with weapons that kill in 3 shots, instant kill-streaks and notifications, grenades flying everywhere, it's just hectic. As such, it looses some key things, like teamwork and strategy, as everyone is going around trying to score a kill before their dead.
With Battlefield (at least from what I've played of Bad Company), it's a much more deliberately paced, tactical game. Its maps are big enough to not feel cramped, but small enough to not feel enormous. This allows for tactical movement around the battlefield. Also, due to the game's slower pace, tactics are more viable, and teamwork is encouraged among others, especially with medics and whatnot. You only feel like your going to die any moment when you start racking up the kills because you're caught in the moment. All that and the sound design is just sublime.
So, yeah, my own personal boycott against Activision notwithstanding, if I had to choose between the next CoD or the upcoming Battlefield 3, I'd go with Battlefield.
[sup]And now I have probably started something I should have...[/sup]
