Mummy or Rectangular? When I sleep it’s usually on my side with one arm above my head. I think I move around a bit, usually rolling from one side to the other a few times a night. And when I’ve camped before, I find myself “cocooning” in the morning to keep my head warm.
Sooo… A mummy would be warmer, take care of the cold head issue; but would I have an issue with not being able to move my arms? A lower level Big Agnes would leave enough room for rolling around otherwise. Never tried a mummy bag.
There are low temp rated rectangulars that people seem to like, and I could always wear my wool beanie to sleep; but they don’t pack up as small. On warmer nights, they can be opened up and used like a quilt.