I've used both as a top quilt and I like the sleeping bag better. I move around a lot and the top quilt gets drafty in cold weather. I have a full zipper bag and I like being able to zip the bag up to whatever degree I need to keep warm depending on the weather. My sleeping bag is constructed in a way that allows me to shake most of the down to the top of the bag and what is underneath me is mostly material. So I'm making best use of the down too.
From answers so far, looks like it comes down to personal preference.