You might want to get the best of both worlds: the WM Mitylite. It's a hoodless 40-degree down bag that has 2 zippers. You can leave the foot zipper closed, and completly unzip the side zipper to get a quilt with a footbox, open both zippers for a summer quilt, or zip the whole thing shut and cinch it around your shoulders like a sleeping bag.

I've been in the process of converting to a quilt since last fall. Just to see what would happen, last January I used the Mitylite as a quilt in my back yard on a 15-degree night in a Hubba tent on a full-length Prolite Plus pad. I slept in heavy long johns, a down sweater (no hood), socks, gloves, and heavy balaclava. I stayed warm below 25; as I recall, it was about 21 when I called it quits and came inside.

I want to repeat that experiment this winter, with a couple of minor additions: down booties, down mittens, down pants, and a hooded down sweater. My guess is I'll make it to 20 degrees. (I recently started using an MSR Ventra quilt, also rated to 40 degrees; I'm anxious to see if it will go as low as the Mitylite.)

I would hasten to add that this was a test. I would not knowingly go out on a 20 degree night relying on that setup about 8 miles into the backcountry. That's why I want to test the limits - so I can leave myself about 15 degrees for a safety margin, in case it gets colder than predicted. If the system works down to 20, I'd be willing to take it out when the low was predicted to be 35 - meaning I'd still be warm when the prediction missed by 10 degrees, and it actually fell to 25.