I hear ya Roger. There's nothing like learning to make your own. Save money (in the long run) and end up with EXACTLY what YOU want based on YOUR needs. But for many folks, the time, energy and practice it takes to get to this stage is just not their cuppa tea. And the original question is a little bit different as it's comparing two sleep systems from the same manufacturer, same fill and shell, same weight, same baffle height, and same 'size". According to Nunatak's website, their Arc Bag is 61 unches "girth", same overall weight (fill and total +/- 15 grams)and same overall temp rating as their 55 inch girth Arc Quilt of the same length. I think this, more than anything else, is the conundrum. With those kinds of specs, why wouldn't you choose the bag over the quilt, since you can always use the bag up-side-down as a quilt (admittedly with a hood in your face), but you can't zip up the quilt and have a bag + hood with the quilt?
My experience to date (with MYOG stuff), is in the difference between baffle 'height' and baffle 'fill'. A Baffle is just a strip of fabric that separates down chambers and allows greater 'loft'. No where on Nunatak's website does it mention 'loft' for either of the systems, which leads me to believe the Arc Bag must have a lower loft OR a lower down density than the Arc Quilt. How this relates to temp rating is anyone's guess. We're never gonna answer this question by speculating, or pushing theoretical numbers around.
John, two down bags of equal fill weight, with equal fabric and equal total weight will almost certainly take up the same space when compressed. You are the only one who can decide the whether the benefits of perceived comfort or warmth are more important to you. Whichever you choose, please keep in mind that insulating your head is almost unarguably the most important aspect in keeping warm, so if you go with a quilt, you need to match your headwear to the quilt. And also consider that, if you want to get your pack volume down, then down will be superior to all other insulation unless you sleep in a puddle or 110% humidity for weeks at a time. Perhaps you could consider upgrading at least some of your insulating clothing/beanie to down...
Must have cross-posted with Beenay's post above. All I can say is thatpretty much sums up my opinions. Since most of the down quilt makers we're talking about will do custom work, it all just comes down to
1) Working out what is right for you;
2) Communicating this to a custom quilt/bag manufacturer, and;
3) Paying for it.
Number one is by far the hardest, and is different for every body.