For me it is fit, fit, fit! And construction details. Opposite of Bearpaw, I am on the small side so it is hard to find stuff that fits. I never wear a down jacket backpacking so the shoulder strap compression is not an issue. Even when I did winter trips in below 0F we only wore wool union suits and a wind suit when carrying a pack. I have problems with sleeves that are too wide (wind gets in) and sleeping bags too wide in the lower leg area. Big issue also are zipper flaps - they need to be sufficiently large. And I would not have a bag without a collar. I do not think it makes much difference if you have 650, 750 or 900 down in a bag, the bottom warmth is all about your sleeping pad anyway. I have a 750 WM Super Antelope (with dry loft) and my problem is that it is too wide. They say it is a 10 degree bag, but for me it is a 20 degree bag. I had a Marmot Aguille 750 5-degree bag (got stolen) that had a very narrow cut and I really liked it better. I would not buy a bag with dry loft cover again. It adds $100 to the cost and I never camp out in the open anyway because I hate to get my bag dirty.

I am not willing to pay the extra $$ for the 800-900 down. The 750 stuff is fine for me. All my friends like the synthetic "Puff-Ball" jacket - I am really leaning toward that for replacement when my down jacket dies.