These are both fine choices: the 100% cotton fabric is easier to work with, and the 50/50 blend is not far behind.
Cotton is the most widely used natural fiber used in clothing It comes from the soft, vegetable seedpod of the cotton plant. As a fully breathable fabric, it is a great choice for hot temperatures. For most screen printers, the 100% cotton t-shirt is a dream to work with. These t-shirts cooperate with most any ink and printing method, and they hold ink longer than other fabrics.
Polyester is a synthetic fabric, engineered for some of the qualities cotton does not offer. It is a more flexible fiber, making the fabric more tear-resistant. Polyester is cheaper to manufacture, but it is less breathable and tends to stick to perspiring skin. Screen printing on polyester fabric must be done carefully, since fabrics with high polyester content are prone to dye migration. The solution to getting great prints just comes with a learning curve.
The 50/50 cotton/polyester blend has been created with both types of fibers to take advantage of the best qualities of each. A 50/50 blend is both breathable and tear-resistant. It is less expensive than 100% cotton and offers comparable comfort. The 50/50 blend prevents the fabric from shrinking, as cotton that has not been preshrunk is prone to doing. Screen printing on a 50/50 cotton/polyester blend t-shirt can be done using plastisol inks or water-based inks with the proper polyester-friendly additives. The major issue with screen printing on 50/50 t-shirts is the tendency of deep dyes to bleed through light inks, especially with red, orange, and dark green fabrics. It is a common and simple solution to first print a grey underbase with a super low bleed ink.
Both cotton and polyester fabrics have a lot to offer in terms of comfort, durability, and breathability. The 50/50 blend is a fantastic fabric that perfectly offers the best of both. Despite its minor issues in screen printing from its polyester side, there are simple solutions to avoid such setbacks. With both fabrics approved for easy screen printing, it is really a matter of personal preference for style and comfort.