RashkhMy knowledge comes from sleeping bags, but it should apply. Down can be a superior fill and hold better heat, but it won't do anything if it gets wet. Where as synthetic fill doesn't lose nearly as much insulating capability. I believe that down loses its oil counting overtime causing it to not insulate as well [citation needed] but this comes from when I was looking for sleeping bags which was quite a while ago. Regardless, it may be helpful to reasearch sleeping bag fill if you are having trouble with your search into parka fill.