PJ, I wanted to respectfully challenge some of your assumptions. This, of course, only applies to my gear. You may be correct in other cases. I usually sleep in a hammock where I find the quilt clearly superior. When on the ground the case is less clear but still compelling, in my opinion. I, like Mark, use a bag when it is cold (below 30° F) but for 3 season use in the Rockies I favor my quilt.
"quilt, since it has more area needs more down. [i'm assuming here that the underbody area of the m-bag is less than the added widith/area of the quilt.]"
"quilt, since it has more area needs more lt. wt. fabric. "
This morning I measured my WM Versalite Super bag (long ≈ 36 oz) and at its widest it's 60 inches. My No Sniveller (long ≈ 22 oz) is 48 inches wide. The girth at the feet of my bag is probably less then 48 inches. In my case, there is definitely less down and less fabric used in my quilt. The single layer loft of my quilt is 2.5 inches and the bag is 3 inches. The weight of the draft tube and collar, zipper and hood count against the bag. My bag has 2 draw cords just like my quilt. The quilt has the weight of the Omni Tape.
"quilt, since it's cold needs 2-3 underbody straps (needs is too strong a word here, but many seem to say that they're needed when it's cold – unless i totally misunderstand other's Posts on this issue)"
I don't count the weight of my strap (hat clip) because it's counted with my hat. I have not yet needed more than one strap but another one or two would weight less than an ounce.
"quilt needs an extra nice, large, thick down hood, not normally counted in the quilt wt."
Any clothing I wear to stay warm around camp is what I typically wear to bed. My base layer is hooded as is my windshirt which keeps my neck warm. I use a Thermawrap vest (≈5 oz) with a high collar. I would carry the Seirus Quick Draw hat I use, (2 oz) regardless. A recent Christmas present was the JRB Down Hood (2 oz) that I will use to push my quilt lower. I haven't tried it in the real world yet. I would probably count that as part of my sleep system.
"quit req. that the Quilt-er carry a tad more food"
My food requirements aren't that dialed in.
"quilter needs to carry a 4.6-7.5 oz bivy to also minimize cold air entry upon movement. m-bagger may carry such, but doesn't need to (unless in an ice cave???) or sleeping in a floorless shelter & not using a gnd cloth or sufficient pad & gear to keep bag off of the snow floor."
I have an Equinox bivy (6 oz) that I haven't had to use with my quilt yet. If I were really pushing things I might use it. I t is interesting to note that the useable loft of a quilt is never compressed. When I roll over the quilt stays put. When I roll over in my bag, the bag goes with me and the down that was once compressed under me needs to loft to keep me warm.
"in cold weather m-bagger might get a better night's sleep than quilt-er due to unwanted cold air entry during sleepy-time nocturnal motions."
As it get colder this may be the case. I have to admit to not really testing the lower limit of my quilt sleep system yet.
What's the practical lower limit for a quilt system? In other words, what do the die-hard cold weather quilt users on the Forums take their quilt systems down to and what do their systems weigh?
Hopefully one of the die-hard cold weather quilt users will chime in. In my case I need to push the limits and see. The difference in weight between the two systems is not huge. For me it is about the versatility of the quilt and in particular the No Sniveller. Everything is multi-use. The quilt takes me from the upper 60s to the lower 30s in total comfort.
EDIT:I see Mark and I were composing at about the same time.

