A thinner continuous layer will be the most efficient insulation, like a snowmobile suit, since all of the heat you produce has to pass out of it, you can more easily regulate how much heat you keep in it - does that make sense?, vs say a thick coat and little leg insulation. But - unless you have a thick and heavy, since snowmobile suits are made of cordura and stuff, snaowmobile suit, you will most likely be wearing a down coat over that and often a shell over that. Down bibs are the best thing since sliced bread. Mine are at least 20 years old - they're hanging here next to me and they show no wear. They're old $180 REI goretex shelled full zip down bibs and the way they keep a back warm is well - cool. The seams were "seam gripped" years ago. So actually a shelled down coat over a shelled pair of down bibs is the most efficient warm and adjustable winter outfit, which is why mountaineers dress that way. Waist length full on down pants are nice, but you want a long down jacket and maybe a sweater under to keep your back warm.
Jim