I have an undead priest, Devouring plague is kool, a little mana intensive, but good none the less. Combined with SW:P, VE, VT, and MF it's fun. Ill walk into 3 mobs of the same lvl, SW:P, VE, then VT then do the same thing to each of them, I never lose life, it's Hilarious. I cant say much for trolls, Ive never played a troll priest, but I'm bias against undead altogether.
Shad Resist
Cannibalize
Unending Breath
WotF
All of these are always used(except for Unending Breath if you're a warlock, but it saves you mana and regeants). My thing with Undead is all of their abilities can always be used in any situation. WotF for PvP or those bosses that fear. Cannibalize saves you moneyz on food, Shadow resist is awesome for Scholo, WPL, EPL, against Shad Priests and locks, and Unending Breath whenever you swim. As for trolls, their throwing specialization gets wasted on Priests, Shammies, etc.
As for proff's, I think Alch/Herb or Tail/Ench. Probably want Tail/Ench if you're taking him to 70. Some of the endgame stuff is awesome.
Hope this helps.