Here is the simple answer: Cotton is natural, polyester is 100% chemical... Cotton is warm in winter and cool in summer, polyester is just hot all year long. Polyester is harder to sew with and get a nice crisp seam when pressed as the (basically plastic) fabric will try to spring back to where it was. Cotton, once it is pressed flat, it is pressed. Dyes in cotton are applied, polyester, they are put in the chemical before they are extruded.
Personal preference, cotton all the way!