My self support happens out of my IK, rather than a hardshell. Because my drybags get so much wetter than they would in a hardshell, I subscribe to the "watershed or bust" theory of packing. Nothing else is really a drybag, they are splash bags.
With that said, I prefer down because it is warmer for the weight and more compressible. Trusting my drybag lets me go that route. I do like a waterproof breathable shell, and typically pack my 10 degree bag for fall, winter, or spring trips. In the summer, I'll use a 20 degree bag with just a nylon shell. A 40 degree 1 lb down bag for those perfect weather ultralight trips.
but if you aren't 100% sure your bags will be dry, take a synthetic bag.