Sometimes down bags are NOT a good choice. Like here in Alaska on a ton of hunts. Constant rain or a float hunt are no place for a down bag. If down gets wet, you cannot dry it out in the field, and you may freeze. Synthetics are a far better choice in wet conditions.

As for -20 bags, I have a -40 bag that I use for moose and caribou. Usually it does not get much below -10 or so where I hunt, but I hunt late in the season when its pretty possible to have a realy weather event. The -40 bag gives me some confidence in staying out long and in adverse conditions, and sometimes that makes the difference between a moose or no moose.