Topic Rolling mage. What race for horde?
If you want to pick based solely on racials, I'd say it's a tough call, and it depends on what you're going to focus on. Otherwise, I would definitely pick what you think looks best. I went with Troll for the looks.
Think about the number of hours you will be watching your toon stroll through the world of Azeroth. I think that appearance, emote's etc are also important. Pick something you will be happy with for a long time to come. I picked troll for both racial abilities and style.
undead racials rule
cannibalism and underwater breathing are very handy for solo questing, and a little shadow resistance and dodge doesn't hurt a thing.
also have to love the casting animation as mentioned above ^^^^.