Quilts are usually a couple of ounces lighter than a bag of comparable rating. For bags that are true 20 degree bags, the difference between a bag and a quilt can be around 10oz (eg. compare the WM ultralite to JRB quilt).
For 30 degree bags/quilts there is not much difference in weight, often only 2 ounces or so. You can save a bit more if you have the $$$ for a Nunatuk quilt but the weight savings are still seldom more than 4 ounces if you are getting, say, a WM summerlite. If you need a larger bag (eg. megalite) then the weight savings by using a quilt are more attractive.
So for 30 degree bags/quilts, it is largely a matter of preference according to how you like to sleep. For colder 20 degree conditions there are more weight savings to be had… but having said that, many people prefer a bag below 30 degrees anyway because they want to completely eliminate drafts in those conditions.