They really don't look that much cheaper to me than the montbell, and somewhat heavier.
SD Arrow Rock 30 30* 16oz of 600 fill 2lbs 5oz $200
MB SS Down Hugger #3 30* 12oz of 650 fill 2lbs $200
Granted, I compared the cheeaper MB bag, but I wanted to look at near equivalent fill powers. It looks like the biggest difference is the amount of down (600 fill vs 650 won't change that much here I don't think). If the SD is just as good as MB at eliminating cold spots with their stretch tech, then the SD bag should be warmer, but that is an if. MB usually are rated pretty close to their stated rating. But, its a little unusual to find MB bags on sale, maybe lesser so with the SD.
EDIT: MB regular fits to 5'10'', SD fits to 6'0''. This might also account for the weight difference.
Edited by drow42 (01/25/08 05:57 AM)