In the current environment of 29's if you make a glass cannon on horde all you are going to be is glass, on alliance side you might have a chance to be cannon some of the time.
Let me explain that, horde lacks communication and focus fire, also lacks the brain power to pick targets in many cases. Typicaly horde has alot more randoms lately that don't join vent, alliance does not. You play a fire mage alliance side and more often than not you will be able to get in at least a little free casting, you try it as horde and everyone on the alliance team will know you are LOLGLASSCANNONFIRE before the gates even drop and they will prioritize you accordingly, you will die in a global or you will get a bop and then die in the global after you lose the bop.
Further the best race I can think of for a fire mage is nelf, and that's alliance only.
So yes faction and faction balance is actually relevant to the question.