Cotton tees are comfortable as they are softer and breathe easier than the ones made of other fabrics. The downside is that they will shrink and fade. Which are key factors to consider when buying a shirt. So purchase a size larger at least.
Polyester T shirts.
Polyester tees are will never shrink, wrinkle or fade. The fabric does not breathe. The fabric will out last cotton being synthetic, so washing is not going to ruin the shirt anytime soon. The shirts are not as comfortable to wear as the cotton tees are by any means. And the synthetic fibers are not as ink friendly as cotton.
The 50/50 blend tees do not shrink like the cotton only ones do and they breathe a lot better than the polyester ones alone (but still not great). They also wrinkle less as well and have a better shelf life as opposed to the plain 100% cotton t shirts. However, they are not as comfortable as them either.
Cotton Tees Vs Polyester T-Shirts Vs Blend Tee Shirts.
Overall, for comfort go for cotton preferably organic if you have the extra coin.