I have the base game and Tomb of Ice and I'm trying to figure out if Altar of Despair or Well of Darkness would be better to get. I've played with both sets (friends) before, but he is having a hard time remembering which had the better cards. My end goal is to buy Sea of Blood and 1 more set and start a campaign. Which of those two adds the most skill cards, treasures, and other things that would be useful to a campaign setting?

I don't know the exact component counts as far as which has more , but as I recall they were both pretty close. You could check the rulebook pdfs in the Catalog section if you want exact numbers. There's a component list at the front of every rulebook.

I would personally favour Well of Darkness. I like it better on the whole. Ferrox, boulders, etc.

Altar of Despair would be better if you plan to be a rat-bastard OL, since it's the one that gives you Dark Glyphs, Trolls (the only monster I've known to one-shot an entire starting party, although it did require a very specific initial draw of OL cards, IIRC) and Deep Elves (Frost is nasty.) Of course, AoD also gives the heroes access to Tahlia and Kirga.

Quick answer:

With the advanced campaign rules of RtL or SoB, I'd say you want WoD

For standard games, I'd prefer AoD.

Long answer: http://www.boardgamegeek.com/thread/556885/which-vanilla-expansion-is-best

AoD all the way!!

Cooler monsters (and more of them) and niftier heroes.