Whatever bag or quilt you choose, it will need to have at least four inches of loft above you, that is, a mummy bag will have 8 inches of total loft. Most manufacturers don't tell you how much loft their bags have, but if you're buying in person you can see for yourself. Their temperature ratings are deceptive, as they sometimes only list the "survival" rating, which is much lower than the "comfort" rating.

I have only limited winter experience, but I have slept on the snow at zero degrees in a Trailwise Slimline bag with 7" of overall loft, 60 percent of which is on top, so about 4". Wearing long underwear, wool pants and sweater, down booties and stocking cap, I was warm enough, or would have been if I'd had an adequate pad (I had only a 3/8" three-quarter length pad.) I'd suggest taking a self-inflater AND a closed-cell pad.

Down is the obvious choice for low temperatures, as there's not much liquid water around, but it's expensive.