In reply to neuromancer:
Not massive differences but basically the Alpine is about 250g lighter and offers one big storage compartment with a single top opening. The Mountain is heavier on account of side access and the extra convenience that brings and a burlier hip belt sporting gear loops. In the bigger size 45:55 the Mountain also offers a split main compartment and an extra side pocket.
So it's convenience vs weight. Where do your priorities lie?