I've tried the foam pad on top and under my inflatable, and for me it's definitely warmer and feels better with the foam pad on top. However sometimes I bring the foam along in the summer and I'll put it under the inflatable because it will keep it from sliding around.
But it sounds like one of those things that people have to try for themselves. Some people, like Mr. Llhikers, like the CCF on bottom for protection against puncture.
In life there are no limitations. Except stupidity. If you're stupid, you're screwed.