Neither class is terribly mana efficient.
The mage has evocation, and will want some of the arcane talents for mana efficiency.
I've go with a frost mage, over a fire. More control at the cost of a little less burst.
But you get a lot of talents which reduce the cost of spells.
The warlock has lifetap going for her.
I believe this class is the least efficient from a mana perspective.
Lifetap is extremely powerful, and you'll have passive healing from the party/totems.
Still Lifetap when you cannot otherwise cast, or you'll kill your dps.
I'd say go with whichever class you enjoy more.
In the end, it will work out.
Your party will have some form of replenishment.
You have passive mana regen either from Mana Spring or Blessing of Wisdom.
You can stack some mp5 gear if you want.
Both classes bring things to the table, and no matter what you pick now, Blizzard changes the game.
They've specifically made class balance threads recently, so we can expect changes are coming.