The entire combat and character development system is going to be changed in a few weeks (CR 2.0), so maybe you shouldn't care about this anymore.
Here are the main purposes of attributes:
perception: unarmed close combat (martial arts)
focus: stealth, gunman ranged
reason: hacker, coder
vitality: max health, health regeneration speed
belief: hyper-jump (forgot the main purpose)
I don't know which server is most populated (guess they're kinda equal). According to what others say, Recursion is "community" (quite many player events there), and Syntax is "Role Play" (RP). However, officially, the only difference is in Vector which forces you in PvP once you hit lvl 16.