As the FAQ here is in desperate need of updating, I reluctantly went to the o-boards to see if I could find a leveling guide there. They have one stickied, with a particularly lengthy debate on whether affliction or demonology is a better tree for leveling. Generally the consensus seems to be that Affliction is better for normal quests due to the minimal downtime, but that demonology is better for group quests because you can solo them.
From your experiences is this true? Or is the downtime of demonology comparable to the affliction tree? For the last couple of levels I've been drain tanking in Zangarmarsh and I'm quite enjoying it, but the idea of soloing group quests intrigues me as well, so if affliction does indeed have a lower downtime I may buy dual spec for my lock.