I'm wondering if either faction has any positives or negatives to consider in any aspect for the mage pvp experience?
Yes and no, play what you like the most, you won't have third chance.
Playing an Undead Mage:
Pro - Will of the Forsaken against priests, warriors, and warlocks.
Con - Gnome rogues and warriors can Escape Artist your roots and slows.
Playing a Gnome Mage:
Pro - Escape Artist to get rid of slows after you root warriors and rogues if Blink is unavailable. You also have additional intellect to deal more damage.
Con - You'll be tag-teamed more often in world pvp. There are no real Horde racials that would prove annoying for you to play against.
If you're playing Alliance the best race for it is a gnome,
If you play Horde then the best race for that is undead.
Gnomes have more intelligence as a racial buff and Escape artist to cancel out snares and traps... that gives you an extra move when blink is on cooldown. You're hard to click but would still be targeted a lot anyway because of the danger you pose. Arcane resistance 1%.
They also have a bonus to engineering which you can throw into your rotation.
Undead have something similar to escape artist that removes Sleep, Charm and Fear. They can also eat gnomes to restore their HP. Shadow resistance 1%.