Battlefield and Call of Duty play a lot differently. Call of Duty is basically a jack of all trades, streamlined multiplayer shooter, whereas Battlefield focuses on objective-based modes on huge maps, with vehicles. No doubt, Call of Duty is going to have a larger player-base, but it's hard to say which will be better considering how different the two are.
I'd probably prefer Call of Duty cause, what can I say, I like just killing people. But I think in general Battlefield will be a better game all-around.