There are many variables, but what will result in waking up warmer on a cold night - the shelter or the tent?
Agreed that on a wet and windy night - the shelter normally allows you to stay drier unless the wind is in the wrong direction (which I am positive the shelter builders designed to be so)
It would appear to me that the wood of the shelter floor would be warmer and conduct less heat away from your system than snow for argument. But then I always "seem" warmer in a tent?
Any thoughts or even science?
BTW Blood mountain shelter manages to keep a - 10 deg F even in summer.............