Personally I hate the cold. Full length all-the-way for that reason alone.
As to the temp range, well that depends on how warm/cold you sleep, where you're gonna sleep and how often you're gonna sleep there.
I like being warmer than I do colder, again, I hate the cold. I would rather have to wake up to vent than wake up shivering or clamoring for every extra 'anything' for insulation to help keep me warm.
Are you going to be sleeping in the cold often, or just often enough that you could bring warmer clothing?
Is weight a concern? If not then the extra few ounces of insulation between the 0/20 may not matter, then again it might be completely wasted if you never need it.
If it were me, and I was planning on sleeping in cold places often I would buy the full/0.
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