Right now, mages are the kings of DPS. As far as utility, that's up in the air of opinion.
Warlocks offer portals from a location, healthstones, indirect rezzes, and fear.
Mages offer portals to a location, mana (crack), and polymorph . . . if I recall.
Personally, despite what you'll hear, I recommend warlock. Unless you're certain of a group all the time, warlocks still solo things most mages wouldn't dream of taking on.
Demonology is no joke if done right.
But, for a 5-man raid? Yes, mage. Just hope your tank knows how to hold aggro.
Just my humble opinion.
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Sanctimonious know-it-all.