Personally, I'd probably take 3 Spells from the Lore of Dark Magic and 1 from the Lore of Death. With the Lore of Death, you are guaranteed to get at least 1 good Character snipe spell (Spirit Leech) when combined with her Lv4 and D3 added to her casting roll, makes it quite easy to get off. The other 3 Spells from the Lore of Dark Magic all benefit from the D3+1 (for Hekarti's Blessing). Since Shadow and Dark Lores are both VERY similar, I would opt for the +1 to cast.
Doombolt is a GREAT magic missile. For that reason alone, I think that the Doomfire Warlocks are worth taking (Not to mention that they get a 4++ for a fairly cheap unit). Two Doombolts are sweet. Word of Pain, Bladewind, and Black Horror are all good spells as well. The others...situational.
My armies: Empire, Dwarfs, High Elves, Wood Elves, Dark Elves, Lizardmen, Skaven, Tomb Kings, Ogre Kingdoms, Orcs and Goblins, and Skitarii (40K).