My wife has zero circulation in her body, and can have cold hands wearing a sweater on a 70 degree day sitting in the sun. We got her a -15 degree down mummy bag, and it works for her. We often camp in the desert in the fall/winter/spring when the temps drop into the low 30s or high 20's. We also summer camp in the Sierra's around 10,000 feet or higher, with similar night time temps.
There are gazillons of types of bags, but I haven't heard anyone talk about some of the factors to look at in a good bag.
1. For women, a bag should have a small foot box. The larger the foot area, the more area her feet need to heat up, or in my wife's case, the less able she is to keep her feet warm at all. Good bags add extra insulation in areas like the footwell.
2. An excellent draft stop/zipper shield. A lot of heat sneaks out the long zipper. Having a nice down (or synth) baffle the full length of the zipper seam is very important. Also, it should have a canvas like (or denium) material along the edge of the zipper to prevent snags.
3. Neck gaskets. Its important for people that are just too cold to be comfortable to have a proper gasket or baffle above their shoulders, to keep out the air from around their face. Having a separate draw-cord on the baffle is useful.
4. Insulated hood with drawcord. A significant amount of body heat is radiated out of the head. Stuff your head in the hood, draw it tight around you, and stay warm.
5. Look for comfortable materials inside the head/neck/hood/baffle area. These touch your cheeks and skin, and can be irritating if not high quality.
6. You should be able to unzip any zipper, and undo the drawcords from within the bag. A good bag has inner zips with nice large pull tabs to enable this.
7. hip fit. Most women like a roomier hip area, and that smaller foot box mentioned above.
8. his & hers bags! Don't think that what works for you will work for her, or vice versa. Get a bag tailored to the person using it. For me, I create ample heat, and stay pretty cozy in low 20 temps with a 35 degree bag, wearing a t-shirt. Should I ever feel a chill, a thin liner works awesome. Also, I'm quite large, so a mummy bag just won't work for me. I use a big rectangle, and I like it.
9. Pads. For the first few years, we had nice comfy air mattresses, but she would always be freezing cold. We then switched to
pads like this, and have been significantly more thermally comfortable, and remarkably comfortable physically as well.
For me, what worked for us was to buy about 10 different bags from Cabela's, then try them out and return what didn't work. I find
Cabela's site pretty easy to use, with a great deal of info on the bags.
I would also agree with most people that sleeping on a cot is going to be less comfortable/warm than on an insulated ground pad. The only exception to that might be if you are older and/or arthritic and getting back up off the ground poses a problem.
The more comfortable and happy everyone is, (and particularly the wife), the more relaxed and enjoyable, and possibly longer the trip can be.
