Another vote for Western Mountaineering. I have both the Ultralight (with the draft collar) and the Summerlite, neither of which I could really afford when I bought them.

Another advantage to the WM bags, besides their quality, is the fact that they stand behind their products. The first time I used the Ultralight, I managed to snag a seam on the stick that was supporting my Ray-Way style tarp tent, and by the time I realized it was snagged, I had broken the thread. My own stupid carelessness, which I admitted to WM when I sent it back for repair. They quoted me a very nominal fee to repair it, but then they fixed it for free, saying, "Well, maybe the seam was loose." Pretty good customer service.

I think the Ultralight would be fine for me at 20 degrees, although I haven't used it in weather that cold. I used the Summerlight (rated 32 deg) at 24 degrees last September, and can tell you that this is a very bad idea. It's also a bad idea to place too much confidence in weather forecasts.