Others may chime in, but here are a few:

Down is lighter and compresses more.
Warmer than synthetic for same/less weight.


Synthetic is generally cheaper.
Easier to dry.
Can still provide some warmth when wet (i.e. not totally soaked).
Bigger selection and price range.
Can loan them out to others as needed (you generally won't lend out a down bag - at least I don't).
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