Poly is warmer. I use warm and natural for quilts intended for late spring through early fall AND for kids (they always seem to be too warm, they complain). For everything else, it is 20/80 or poly. For a super, super warm quilt, you could always get a thick batt and tie it. Joann's used to make a great thick batt, but 8 months after the product came out, they cheapened it. It isn't even half as thick. Instead, I get the poly on the roll at Hancock's. For someone who gets very cold due to health issues, use a double thick. Harder to tie, but oh, so warm.
Keep in mind his preferences. Some people want warm without the weight. Consider also that thermo batting used for clothing. I"m making my bro a quilt this year that uses that.