I think on average there are more xbox players aren't there? Especially when you consider the agreements in place between microsoft and the COD franchise. Overall I think there are just plain more xboxs sold than there are PS3's and more younger kids or early teens that end up going the xbox route. Consequently if there really is a larger and mostly younger playerbase to draw from, you could probably surmise that there are more bad or just very average players helping a great to stand out more.
Personally I can't attest to this as I just have PS3 and have for some time. Just a theory I've sort of heard other people touch on and one I kind think might not be far off base if at all, it does come from players who've had quite a bit of experience with the same games on both platforms tho.
The 'sexbox' does seem to have a much larger fan base from everything I see and hear, even the nick name. It also has the draw of great online game play since it's a payed service. Probably a larger, younger playerbase. Personally no prefference, don't see any reason to dislike either.