Troll- mana regenration, and increased spell casting.

Undead- basicly can remove fear, charm, sleep ( pvp mostly) thats about it.

also if you say something not related to this im just going to report/ignore it. idc if you think this game sucks or w.e im tlaking about troll and undead priest not another class so dont be an idiot and say Mage or w.e.
yes i do know that spell casting is a racial i just didnt wana say it was because not many people now a days even know wtf "racial" is.

and the mana regen i think.. is like 1% only

i think i might go UD since they made the game so easy last time i went OOM on my holy priest constantly even with good tanks and knowing how to heal. so i dont think mana regen would be an issue. nor casting speed.

undead always made best casters
as a preist the need to remove fear charm and such quickly comes in handy
with the right spec mana regen is not a problem later on in game
provided they did not nerf it like so many other things

The increased spell casting you get from trolls is a racial that can only be used once every 2 mins i'm pretty sure. If your making the decision based upon PvP undead is the way to go. I play a priest as you can see from my avatar, i'm on alliance now but i use to play undead on horde. The remove fear/charm/sleep is worth it when it comes down to arenas.

Happy Gaming.

Edit, if your ooming that might be because you're holy, Try disc. It's 100% better.