100% cotton is exactly that – 100% cotton.
50/50 is a mixture of 50% natural cotton fibers and 50% polyester synthetic fibers.
While there are pros and cons to each, we always recommend 100% cotton. 100% cotton will shrink slightly more than a 50/50. 50/50 will hold its color slightly longer than 100% cotton. 100% cotton is more comfortable than 50/50. 100% cotton keeps you cooler because it breathes better. Print quality is better on 100% cotton.