I was troll on my mage when I was horde and found it to be amazing compared to goblin, which was my other race for a while.
If you like a more active playstyle than I would suggest troll over pandas, especially for fire since you can pop zerking right before combustion to snag an extra tick or two.
If you are mastery-arcane I would go with Undead and for haste-arcane I would go with Troll.