I have both a bag and a quilt (Arc Ghost by Nunatak). I bought the quilt because for most of my trips I would thrash around in my bag all night, am a side sleeper, and for weight savings. So far, I've not been disappointed with the Nunatak. It's become my go-to for three-season use, as well as packing for my teaching trips where accomodations can range from hotel to open-air shelter. As others have said, a hat/hood/balaclava is needed once you get to the colder temps, but I like the fact that the quilt can be adjusted to accomodate the layers I'm wearing underneath. With a bag, it seems like buying one overlarge to accomodate insulating layers would result in a larger space to heat when you're not wearing them.