As of the current patch, Warlocks deal more raid DPS and are significantly more successful in the high end 2v2 and 3v3 arena than Mages. However, Mages much more common in the 5v5 arena. Both do sufficiently well in BG's.
However, these imbalances are not what you should base your decision upon. PvE damage rankings between classes change often throughout the course of patches. For example, Warlocks were among the highest DPSers during the early raiding days of WoW, but eventually became the worst as other classes obtained better threat management and sustained DPS. Thanks to synergies with the (now practical) Shadow Priests, larger debuff limits, and other factors, Warlocks are now among the highest raid DPSers in the game - this will eventually change in the future.
Warlocks are seen often in 2v2 and 3v3 due to their strength in small battles. We're actually a very versatile class in terms of teammates - we have the talent options for different strategies in the arena, such as an "outlasting" spec with a healer or a "DoT DPS" spec with a Shadow Priest. However, Warlocks have less representation in 5v5 due to the lack of survival options and damage mitigation.
Remember that your role in arena will often be CC and healer disruption, rather than DPS as a Warlock. Mages, Rogues, and Warriors are generally better for actual DPS unless you plan on running a team for DoT DPS.
Subject: Warlock or mage? 
Sat Nov 24, 2007 1:08 pm
