I recommend a Wiggy's bag because I have the Ultra Light FTRSS system (also Wiggy's sweater and poncho liners). In warm weather, over 40 degF, I use the outer bag. At 40 to 20 degF, I use the inner bag. Below that temperature, I use both. (I would probably get the Super Light FTRSS if I had to do it over again.) Wiggy's bags can be stored compressed, and washed in a washing machine!! Because the bags drape to reduce convective air space, they keep me very warm and comfortable. The bags are expensive and weigh, but they really work for me.

Note: My soon to be ex wife threw my Wiggy's bag in the trash. My son let me know, so I went over around 2am and fished it out of the trash, covered with the the evenings spaghetti dinner. Two washings at the laundry mat, air drying, and they seem as good as ever. Think you could do that with a down sleeping bag?

Spud