Gnomes have a higher intellect. Mages require intellect. The 5% intellect bonus given by the Expansive Mind trait is calculated after adding bonuses from gear, so it continues to offer an advantage even at high levels. So, every 20 intellect you get from gear gives you +1 intellect.
Humans get a 10% increase to Spirit, which is alright. I don't know the math about it, I've never been much of a spirit fan, since mages have an endless supply of food and water.
Draenei get a free healing spell, which can be useful depending on the situation, plus a 1% bonus to hit for yourself and your party.
If it were me, it wouldn't really make a difference, I would just pick my race based on which starting zone I know the best of the three, so that way I can get to my level 10-20 zone of choice the fastest.
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"Do you know what "nemesis" means? A righteous infliction of retribution manifested by an appropriate agent. Personified in this case by an 'orrible c*** : me."