The long answer is it depends. I have slept a quilt setup to 0 before but you do have to do draft management. IMO, everyone can sleep a quilt pretty well to mid 30's. With a little knowledge, most can sleep one well to 20 ish. Below that, I think quilts are for some people. A sleeping bag is not perfect either. If the sleeping bag is to small, you will get cold spots from compression (this is the problem I have most often). I'm sort of lanky, and long limbs need some space. A quilt, is less likely to have cold sport due to excessive compression but it is more likely to have draft issues. 
All things being equal, if I had a perfect fitting sleeping bag, I would go with a bag below 20 degrees most of the time. Since I don't , it depends on my mood and other things such as outer fabric of each system.  There are so many variances in sleep systems, it is really hard to have a one size fits all solution. Pad choice matters, pad size matters, and orientation of bag / quilt to pad matters especially as the temperatures drop.