Thickness of the insulating layer determines its ability to ward off cold, in general. The difference between down and synthetics is that for a given insulating value, the synthetic fill will be appreciably heavier.

This matters in a bag used for backpacking, but not for something you will keep in the car. Since I like to backpack and get outdoors a lot, a good (albeit expensive! down bag is worth the bucks. I might as well store it in the car. If I didn't backpack, I would save big and get a synthetic.