It depends on if you want to stretch the envelope or not. If you do, then get a 30 degree bag along with a nice puffy jacket (which you may own anyway) and be prepared to sleep in that most of the time. You have to know your own sleeping and hiking style. For example, do your legs or feet get cold at night? If your feet get cold, you will need to bring something extra (as your feet are probably warm in your shoes). If your legs get cold, then you have to ask yourself if you bring leg insulation for the daytime (long underwear or insulated pants). If so, then re-use makes sense. If not, then bringing extra clothes just for the sleeping probably doesn't makes sense (might as well get a warmer bag).
If you aren't prepared for that much fussing (which may take some trial and error), then get a 20 degree bag. For the area you mentioned, it is probably the most common one used. As said earlier, the weight penalty is rather minor (once you've made a sleeping bag, it doesn't take much insulation to add extra warmth).
As far as particular bags are concerned, FF and WM make excellent, top of the line bags, so picking between models is probably more important than brand. For models, it depends to some degree on how much room you want. Montbell makes excellent bags as well. I believe their down is not quite as good but still excellent (if FF and WM is a 10, Montbell is a 9) but their spiral and stretch bags are quite innovative. These bags allow you to stretch, while still retaining a very efficient, warm bag. I own a FF and Montbell stretch bag and I'm happy with them both.