After having taken both my Sentinel and Guardian to 50+, I feel Guardian is the AC for me.. Simply because the rotations come out more naturally, you can flip between tank and DPS at any time and of course, the force push.. such an amazing utility move in PvPs, while holding off 2 vs 1 in defence. For some reason, my sentinel feels squishy at level 55, probably because he's not equipped well enuf, but he had always had problems in PvP. With the guardian however, PvP is a lot more easier I feel.
Snipers are still the pain in the backside though!