I myself never did that well in competetive PvP (mostly due to being a very casual PvPer), but I really love both SPriest and AffliLock.
I'd say pick a SPriest if you like healing as an option (Holy and Disc are very fun to play! esp. holy).
And pick Lock if you like 3 completely different DPS specs.
(I myself just started playing on EU, leveling priest and want to make lock for WoD as well)