Vlad offers a Mage's side, but I'm here to vouch for the vastly superior Warlock.
Warlocks offer the versatility of raid buffs, raid utility, and are just straight up cooler than their inferior step-sister, the Mage. The age-old tale is anything a Mage can do, a Warlock can do better. Each Warlock offers a different playstyle, and oddly enough, you don't even have to play with a pet anymore (it's actually beneficial to not use a pet!)
Affliction is all about draining your opponent's soul and empowering yourself to slowly, yet powerfully, wittle away at the strongest foe's health. Affliction is currently simulating at the top of the charts over every other class due to it's easily sustained rotation, quick burst, and high multi-target DPS capabilities. Multi-dotting is an exciting thing to do in raids and dungeons rather than Multi-shotting or using Blizzard. As Affliction, you'll take Grimoire of Sacrifice, sacrifice your pet, and do incredible damage alone while gaining an ability of one of your pets. You can choose between a Shield Wall type ability, an interrupt, or a magic dispel! Affliction feels very fun to play and the learning curve is nowhere the level it used to be.
Demonology Warlocks bring the fun of turning into an Illidan look-alike and using powerful demons to take down your foes with very little, if any, discrimination. Imps, Felguards, Infernals, and Doomguards will be at your calling while you turn into a powerful demon to slash at your foes and perform some of the highest AoE capabilities in the game. With your cooldowns, you can have 5 Imps, 2 Felguards, and a Doomguard out at the same time. You'll make Death Knight's Army of the Dead look like a Mage spell.
Destruction Warlocks play similarly to a Fire Mage, but way cooler with the incredible Chaos Bolt! Here again, you'll use Grimoire of Sacrifice to empower yourself as you quickly and powerfully burn your opponent's life away by using hard hitting fire spells to charge up the use of Chaos Bolt. Unfortunately, Destruction isn't performing as well as most other classes at the moment, but that's likely because Affliction is at the top.
Mages bring Timewarp, which can also be supplied by Shaman's Bloodlust.
Warlocks bring a battle ressurection (Soulstone), amazing Healthstones, a new portal system to help raid members mobility, and 5% extra magical damage on enemies brought only by other Warlocks, Assassination Rogues, or a Hunter pet that no Hunters like to use. If you had to choose between the two, I think the choice is clear.