Choosing a sleeping bag

I find a bag significantly warmer than a quilt by about 10 degrees worth. But I am also a restless side sleeper and the rolling around creates a lot of air exchange, and I think if you sleep on your back and don't roll around a lot (ie: non-restless sleeper) then the difference is minimal.

However, I mostly use a quilt because it's completely non-confining nature allows me to sleep a LOT better, plus it's much easier to get in/out of.

A bivy will add 3-5 degrees of warmth, but a horizontal zip will make it harder to get in/out. A side zip going down about 1/2 way fixes that though :)

I also notice that when it is extra cold, wrapping the quilt under me (or using a bag) is warmer even though the insulation is compressed. I think it's because it fills in the contour gaps. However, I can be just as warm by using a quilt rated for 10-15 degrees warmer than the bag and enjoy the non-confining sleep.

The short answer is “It depends”. If you have a warm hat, aren’t a restless sleeper, and and using a bivy sack, then a quilt with will be just about the same warmth as a bag and lighter weight assuming both are made from the same materials.

Some people love quilts because they give more freedom. Some people like bags because the sleeping bag surrounds them and gives them a cozy feeling. It’s really personal preference. A few more thoughts on quilts on my quilt recommendations