Style.
Mage is a glass cannon, and personally i find it on the boring side of gameplay, with each spec basically being color/graphic pallet shifts of each other.
Warlock specs are very different in playstyle, not just method. Pets are strong enough to make questing and farming/soloing/groups much less of a hassle. I've seen mages do similar things, but they have to kite so much its rather annoying. I can send in my pet, stand still and kill things.