Topic Undead or Blood elf
TBH, neither of those races have very strong racials for PvP. WotF shares a 45 sec CD with PvP trinket, and while Arcane Torrent does silence enemies and return mana to you, but with a 2 min CD it's barely anything to brag about. If you want racials to be some use, roll a troll. If you don't care about racials, I can't really tell why would you ask us because it's purely about your personal preference.
UDs (both male and female) do have some impressing spell animations though.
WotF is very strong, specially vs other priests in arena.
Roll undead if you go PVP, belf (if you rly want belf/undead) for PVE