Definately UD if you plan to PvP at all, especially in 2s and 3s where warlocks are abundant. If you like running around helpless for 10 seconds(longer if they chain it) while your partner dies, bmg don't roll a UD. You roll UD for WoTF, Devouring Plague is a just a bonus.

In 5s it is debatable, if you run a 4 dps setup then go UD for the extra dps. But if you run a balanced team with 3 dps 2 healer then a troll priest might be superior here as you'll be expected to mana burn alot and the haste will come in handy. WoTF is less useful here as you'll most likely have dispels coming from a pally.

Forget about BE, as a priest you should never put yourself in the open for a crappy 2 seconds silence. In 5s you'll get cut up so fast its not gonna be funny, in 2s and 3s you'll get CCed or mana drained to shit.