You mean race, not class ;P
Undead are definitely best for PvP, with WotF and Devouring Plague. Both of those have limited use in 5-mans and raids (WotF being better for healing if you get feared, Plague better for a shadow priest for some extra damage). Cannibalize helps downtime a bit in both PvE and PvP.
The elves (while I don't have one) appear to be better for PvE. The extra silence is good everywhere (PvP, soloing, 5-man trash) and the two abilities for extra mana are also excellent for 5-mans and raids, especially for a shadow priest who struggles with mana.
Personally, as an endgame shadow priest, I wish I could trade my undead for a blood elf. For PvP or casual play, I think Undead are probably the way to go. Like you said, though, it's really close.