6 of one, half dozen of the other. I put a mattress pad UNDER the mattress. It modifies the heat to a gentle warmth so we aren't jiggling the control all night. Heat does rise and the thermal mass of the mattress distributes the warmth.
Another note: if you have a window at the head of the bed, your head will be chilled in cold weather. We created a "headboard" with rigid foam insulation and covered it in quilt batting and a matching fabric. Then we wedged it in place over the window. Other cold weather campers have copied that idea and found they sleep better.
"The only time you should fear cast iron is if your wife is fixin' to hit you with it."-Kent Rollins
First law of science: don't spit into the wind.
Magnus