Of course as with all pillowcases and duvets it is important to consider the fabrics and quality of the materials. Cotton is a material that has been used in bedding and clothing since the days of ancient Egypt, as a natural material it is a lot cooler and has a greater degree of breathability than some man made fabrics like polyester.
As a byproduct of the petrochemical industry, polyester is more durable than cotton based fabrics, however it is a lot less breathable, trapping more heat which can make it less comfortable in warmer climates as a bedding material.
Here are some handpicked camo pillowcases with their fabric makeup for you to decide:
51x92cm / 20x36in
50% Cotton / 50% Polyester
45x45cm / 18x18in
100% Polyester Blend
66x50cm / 26x20in