This is what I have been told about poly & cotton batt. I am no expert, but this informations has helped me plan my quilts.
Poly Batt does not breathe much and the body heat is reflected--therefore, it tends to feel 'warmer'.
Cotton or Cotton Blend Batt breathes, letting some of the body heat escape.
I tend to think of the batting as to where the quilt is going to live, example:
Hot Climate=Cotton or Cotton Blend, Cold Climate=Poly or Cotton, and have used both in a couple of quilts.