Good point - I guess I got tunnel vision when the OP said "bag."

Quilts are a good three-season choice. I have a WM Mitylite; it's a hoodless, full zip down bag rated to 40 degrees. At 50 or above, I zip it open down to the foot (I leave the foot and about 6 inches of the side zipped, to form a foot pocket), and use it like a quilt, with nothing under me but my sleeping pad. It's a very comfortable way to sleep, and I've never gotten cold.

Below 50, I switch to a WM Megalite (below 30, it's an Alpinlite), which is a full hooded bag, used the traditional way. I think you could use an appropriately-rated quilt at those ranges, too. After all, the down in the bottom side of the bag is compressed to nothing, so it doesn't provide any warmth. For me, it's the hood that is the sleeping bag advantage - and I've often wondered if wearing a hooded down jacket to bed might not accomplish the same thing. I'll have to try it sometime, in the back yard.

Below 40 degrees or so, what's underneath you is as important as the bag above you. Regardless of whether you use a bag or quilt, if your sleeping pad isn't providing enough insulation from the cold ground, you'll sleep cold - and a thicker bag or quilt won't change that.

Enough rambling - I'd second the recommendation that you look at both bags and quilts, and choose the one you like best.