I made a sleeping bag for my wife and went with a flat hood. Mostly because I'd never made a sleeping bag before and wanted it to be as simple as possible. Also because she tends to sleep on her stomach with her arms under her head, a 3D hood would have been constricting. I did make the hood fairly large so that if she cinches it up it will enclose her head better but so far she hasn't used it in temps where a hood is all that necessary (nor do I think she ever will, which is why I made the bag in the first place rather than just buying one).
The biggest lesson I learned from the whole process is that I need to do a better job selling my wife on the idea of a quilt because I'm no good at sewing zippers, especially when they're five feet long.