All depends on your play style do you like to have something to take damage for you while you drop dots and watch or do you want to nuke a target to death before he even reaches you plus mages can solo pretty dang good especially frost mages aoe grinding ftw if you are more group oriented though i would go for mage ive been in groups where they boot the lock thats 3 levels higher than me just to have a mage. Mages also have free food and water free mana pots essenctially and they can pwn in pvp though not as much as locks. A good mage can out dps a lock if they do the right stuff this includes aoe damage of course. Basicly if you want to solo roll a lock if you want to do group instances and stuff go for mage.