Just got my REI dividens, and my 20% off coupon and was going to by a Mormont Helium, but now looking around I see that a lot of people like the Western Mountaineering. The two WM I was looking at were Alpinelite, and the Versalite. How duriable are the outer shells, are there ratings for comfort or surivival, are they worth the extra money over the Helium, ect? Thanks.