I'd actually say Altmer, since their Highborn power is quite useful, and the +50 starting magicka is very helpful at low levels, since you can focus more on health to make your starting character less vulnerable.
It will also make Diplomatic Immunity a lot easier, since you can dress in Thalmor Robes and pass through freely without having to fight anyone.
Depends on the type of Mage, really. A Pure Caster would be better suited as an Altmer, while a Battlemage-Type character should be Breton. High Elves get the additional 50 points, which as Copper said, helps for earlier levels. Breton get the %25 Magic Resistance, + The Dragonskin power, which adds an additional %50. Or more accurately, Spell Absorption. I think it's the same thing, but it also works to regenerate your magicka. Combined with the Atronach Stone, you'd be a spell casters worst nightmare. I covered more on the Breton since Copper covered the Altmer.
My personal choice is Breton. I can't force ever force myself to do a purist playthrough. I always use hybrids. And besides, elves look weird, imo.
Bretons are versatile, like argonians, the absorbtion and magic resist make Breton very goodd at anything anti-magic
Bretons have a magic resist THAT is very helpful on higher diffuclties you will die more from magick then you will die melee (unless forsworn, those guys are a paintrain with no brakes)
Altmer is really powerful early game, but the extra magicka becomes negligible later on.
Breton's power is useful at all times and even if you do a werewolf transformation or VL transform.
Plus, with Fortify destruction and another spell school gear, any mage can just ignore magicka, really.