Get a 30 degree bag that's a couple ounces lighter than a 20 degree bag, and boost it with the MB UL Down Inner Parka (or similar) that you should be carrying anyways for cold nights.
In the Sierra summer months, altitudes 6,0000'-10,5000', I found a combination of a 30 degree bag + Montbell UL Down Inner Parka to be sufficient. That's including a night in the teens and a night in the 20s. (In a tent) I was definitely chilly on the teens night at about 4am, but it didn't keep me awake.
It really is only a few ounces difference between a 20 or 30 degree bag, so it's your choice. I figure I'm carrying the jacket anyways, so why not use it to the fullest.
As far as bags, Marmot and Montbell both make really nice bags that go on sale frequently, but you can't top a Western Mountaineering Megalite or Summerlite (30 degrees) or a Nunatak quilt, they're just pricey.