Short answer? Top is more important. Snow is a good insulator.

Long answer:

I start with a Thermarest, or Big Agnes insulated aircore (I've had the BA for two years now so use it instead of the thermarest) assuming it gets down to what I count as "cold"
I then put CCF foam on top. At "really cold" I usually also put a ccf pad underneath. but really just a thin one to insulate the bottom of the BA. My typical setup looks like a thin yellow 25" wide ensolite foam - then the BA - then a 25" wide blue CCF foam pad - and that's almost as comfy as a hammock. and a lot warmer.

Personally I don't think the bottom layer provides a lot unless I'm sleeping on very hard packed snow or ice that won't conform around the pad. if the snow is at all soft, the snow
provides an excellent insulator, but I like the bottom ensolite pad to keep things from getting too skooshy, protect the insulated pad, and insulate it a bit from a hard surface that (cold) air will get under the pad more otherwise. It's also much easier to peel up frozen than the BA <img src="/forums/images/graemlins/smile.gif" alt="" />