Just FYI, it's really hard to pin down which bag to get and those numbers you're quoting are the survival numbers.
a)Spending a night at -5 in a -5 bag is not fun.
b)Type of ground, ground pad and tent have an impact.
c)Bag's insulation material has an impact depending on type of climate.
Generally you'll want down since it can pack really small and is light, but if you're in a wet climate it can take days to dry.
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