For horde:
A lot of the racials are just a minor improvement, you shouldn't feel forced to roll it just because... The races available to horde mages are:
Trolls: 23 Stamina, 16 Intellect, 21 SpiritBerserking (active): Flat 10% haste increase and it can mount up to 30% if you are "Wounded". This doesn't mean you should stand in fire though

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Regeneration (passive): 10% health regen bonus, 10% active in combat.
Beast Slaying (passive): 5% damage bonus to Beasts, handy in ZG and a few bosses here and there.
Undead: 23 Stamina, 18 Intellect, 25 SpiritWill of the Forsaken (active): Activate to become immune to fear, sleep, and charm effects - lasts 5 sec - 3 min cooldown - One of the best racials in the game (if not the most useful). Effective mostly in PvP against Warlocks and Priests.
Cannibalize (active): Increase health regeneration by 200% while consuming a corpse - lasts 15 sec - 3 min cooldown - While mages can summon their own food, Cannibalization can be useful to regenerate health while you are in combat. Also cannibalizing your foes’ corpse after killing them while they are still watching you is really cool
Underwater Breathing (passive): Underwater breath increased by 4x - It may have not much use for a mage, but sure it is a good racial mage-wise.
Shadow Resistance (passive): All Undead get +10 Shadow Resistance - Nice racial against Warlocks and Priests in PvP.
If you don't care about looks and just wan't to get a slight advantage and want my opinion about what race to pick then I would suggest:
PVP: Undead
PvE: Troll
But remember, racials means almost nothing if you are skilled enough.