The likes of CoD has always had maps etc earlier on the 360, hasn't it? I can't see swarms of gamers jumping to Xbox, for the sake of a few weeks wait. I've known quite a few lads who're solely PS3 who bemoaned it, but waited as they weren't too arsed. At the end of the day, it's not going to be available to you or your opponent, so the gameplay won't be changed or unfair when you're playing on-line. Which, let's face it, is all these exclusives are going to cater to.
The only way I can see 360 gaining any sort of superiority is if they do get exclusivity to certain titles, such as FIFA, CoD, GTA etc. That'll never happen though.