I want to be warm when I sleep I feel you can always unzip but being cold and trying to get a good nights sleep is terrible. I know this.... I am in the Army and and shivered a night or two.
I currently use my army bag and although a great bag when you combine the summer winter and bivy the size is too much (although much better than it use to be with the old cotton bags). My son has a cats meow +20 that stuffs down to nothing but not sure about warmth plus it is a little short.. I am 6-3 but i think you can get most is tall or long sizes.
Suggestions on what to look at that is warm and stuffs down small and dries quickly if you happen to get it wet. I am not a sheep hunter so weight not as much concern as space but one usually comes with the other. I do not normally winter hunt but would like the opportunity. Maybe like everything there is no one bag
Also what to stay away from.