Honestly, about the only one that makes any difference is the undead for the 'will of the forsaken'. That gets you out of fears/charms, etc. However, I could not stand looking at the backside of my mage with bones sticking out my arms and legs (not to mention what a crappy tailor I am for leaving holes in my clothes). I rolled an undead once, and deleted it after 10 levels. Pick based on undead for PvP or if you want to do other things, pick whichever you like looking at the most.
Aesthetically, BE mages are the prettiest, but their racials aren't the greatest for a mage: 2 sec silence/interupt aoe, mana drain (not that much), +5 in enchanting. Silence is useful for melee-class BEs since they're right up front on the mobs. Mages survive by pulverizing mobs from the distance, so the aoe isn't useful unless you want to interupt a caster close to you. It doesn't bother me as much, hence I have a BE mage because of looks.
Honestly, just pick the one that looks the best to you. It is your choice, not anyone else's. I am sure I could find 50 more of these threads that have very different views about which race is the best. Just pick the one you want to be. Remember you have to stare at him/her everytime you play.
Being an alliance mage I don't have the greatest amount of knowledge on playing horde, but from a game play perspective I would have to say that the forsaken would be a good option. Besides the fact that they are really... REALLY not pleasing to walk around as ("behold my crafted epics!... wait, what was that about the salvation army?") the will of the forsaken racial is pretty incredible, if your up for pvp, let your jaw fall off and roll undead ^_^
Trolls are good for their berserking mode where you can cast faster (nice when burning down bosses, etc) but I chose Undead: I like the look, cannabalize is nice, and WotF is nice as well.
No really, Undead is the best in my oppinion. They're the only ones looking really badass.
Now really really, Undead have the the best racial skills. Underwater Breathing is not too pownage, though cool. However, if you don't take an undead, you'll curse yourself for that decision everytime you happen to fight a warlock (Will of the Forsaken FTW).
If you're not a warlock, underwater breathing I find myself happy that I have. I can actually have underwater battles and don't have to watch my breath bar for resurfacing. 3 times the breath bar people don't consciously appreciate, but you really appreciate it once you try doing an underwater salvage quest on a Belf or Orc.