Previous post is completely false. Sentinel is better and faster at both. Level as Focus until 20, then respec watchmen until 40. After that, whatever you want.
Guardians take significantly longer and are complete crap til 40 in PVP.
To level via PvE the tank guardian is to prefer, as a tank you get almost instant grp finder pop ups
And in pvp the sent sucks in survivability, because he gets his uber def cd's around lvl 40
guards in vengance is a lot of fun or you can go into focus which is about the same as the sent's focus spec