Notably, during Early Access of the game, you couldn't take the Mountain Road, only the Underdark path. So the dialogue with Halsin (or Minthara) was written before you had an option available here.
I would strongly recommend everyone to take both paths. The whole Underdark area is pretty big and entirely optional, as is the Rosymorn Monastry/githyanki crèche area. Both ways contain lots of quests, encounters and unique artifacts. You'll definitely not want to miss out either one of these, although the game will let you do that.
For the most enjoyable first time play-through, I would recommend the following route so that you will not miss out on anything:
- Finish all of "Act 1", the entire druid grove/goblin camp map which might end with a final battle.
- Locate an entrance to the Underdark. There are several of them in fact, one far more obvious than the others if you follow the main quests.
- Finish all of the Underdark (kind of "Act 1.5"), including the Grymforge map. Do not take the elevator in Grymforge just yet.
- Go back to the "Act 1" map and now take the Mountain pass route. You will end up in the Rosymorn Monastry area.
- After that map, you can either progress to Act 2 by taking the land route from Rosymorn or by taking the Grymforge elevator - it doesn't matter which way.
If you played your cards right, you will by now have some pretty neat artefacts before going to Act 2, some which are good enough to suffice as equipment until the very end of the game:
You could by now have crafted a Sussur bark weapon, several pieces of Adamantine equipment and found the Legendary Blood of Lathander mace. All of these are quite intricate to get your hands on.