Synthetic jackets work better than down as over-everything style if you are hoping to dry out the stuff underneath. Down will keep you warm for a while but if what's closer to your core is wet you can get the down wet from moisture moving out from the wet clothes under the down.
On the other hand, primaloft etc. stays warm when wet.
So depending on what conditions you plan to use it for, you might want to size a down jacket a bit tighter than a syntho jacket.
At the two extremes - I have a generously sized synthetic jacket I can throw on over top of everything else I'm wearing even if I'm soaked, and a pretty trim-fitting down sweater that I usually wear just over a baselayer and can fit a shell over top of...