Synthetic is heavy, wears out in a few years of usage (compressing it breaks down the fibers over time), and doesn't compress as well. Down lasts for decades when cared for properly, compresses well, higher fill power compresses better than lower fp and is warmer (given the same weight of the fill). There are times a synthetic makes more sense but since wet synthetic is a miserable thing (it stays more dry only if you are thinking in terms of what will work better in a very humid environment, dunk both kinds in water and they will be wet and cold) despite the hype...

I am too cheap and go out too often to do anything but down. Spend once, live warm.