If you winters don't get much below 5C I would suggest the 20F. That is -7C whereas the 0F is -18C. So I would save a few dollars and a few ounces and go with the 20F. However if you consider yourself a cold sleeper, then go for the 0F. It is a lot easier to vent an underquilt that is too warm that it is to create more warmth from an underquilt that insufficient insulation for you and the temperatures you encounter.
Good judgment comes from experience, and often experience comes from bad judgment. - Unknown
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