I am levelling this trio and reached level 15 yesterday..

Basically the warrior tanks, and lock deals most of the damage while the priest mostly casts Renews/SWP and an occasional Heal

I was thinking of speccing the warrior FURY (Prot seems a bit overkill for questing)
The lock will be destruction - mainly using shadow bolt and doesnt need mana efficiency as the priest will heal him
The priest I'm undecided whether to go holy or disc - he mainly heals the other 2 after fights and to regen mana for the lock so Shadow seems a bit wasted

Any advice appreciated