Cool to see you on here then - if you aren't still involved in the community you'll be pleased to know that, while Tek9 hasn't had any teams since we bought it a couple of years ago, Steven joined the company with the site and is still here managing it.
As to gameplay, I can't really give any personal experiences with CoD, but ultimately in esports, with multi-version titles, the version played by top players is whatever version tournaments use. After the move to CoD4, before CoD:MW came out, the community got pretty organised and together managed to take control of their scene, meaning that they managed to resist moving to newer CoD titles which are far worse for competitive play than CoD4.
Actually, iirc, there was a bit of a fight back when they first started getting organised between the "stick with CoD4 and screw future releases if they suck" and "screw CoD4 too, we should all move back to CoD2 and pursuade the tournament organisers to follow us" groups. CoD4 won, presumably as a decent middle ground between suitibility for esports and not being as out of date as CoD2.