Western Mountaineering uses 850 down fill while Marmot uses 800 fill. WM is also made in the USA where as Marmot is made in China. Wm is also lighter than Marmot. IMHO I would not spend the extra money on the WM bison over the Marmot CWM because they are both rated for -40 and both have a lifetime warranty. Marmot started out as a sleeping bag company aswell and they also have some of the cleanest down available! As far as quality goes they are both great bags and there really is not much difference.