Well, neither, I’m in the Underdark at the moment. But I’ve completed all the quests marked on the surface map and all but one or two in the Underdark, and I know there are two routes to Moonrise Tower - the mountain pass being one of them, and the other being through the Underdark. Currently my plan is to finish up any quests in the Underdark and then go back to the surface to take the mountain pass route.
EDIT: I guess I just assumed that was the start of act 2 because when I first reached the route to the mountain pass I got a warning to finish up any businesses in the starting area before proceeding. But maybe that’s not technically the actual transition point.