Lol, poor Tasty, you're so abandoned.

Personally though, I think shadow would be the better route, even with having a pally in the group. To go down to sanc aura though you are sacrificing avenger shield. While I don't know how *needed* it is, I certainly like the spell quite a bit and wouldn't want to give it up. 4 Shadow priests = healz and mana return while doing DPS. Not sure if it would be enough to keep a tank up for too long though, maybe in normal dungeons?