It really depends on the specific needs of your project and your personal preference. Both cotton and polyester shirts have their own advantages and disadvantages when it comes to screen printing.
Cotton is a natural and breathable fabric that is soft and comfortable to wear. It is also a versatile fabric that can be used for a variety of different applications, including t-shirts, bags, and more. Cotton is also a good choice for screen printing because it absorbs ink well, and it can create a high-quality and vibrant print. However, cotton can shrink and fade over time, and it may not be as durable as polyester.
Polyester, on the other hand, is a synthetic and durable fabric that is resistant to shrinking, fading, and wrinkling. It is also moisture-wicking and quick-drying, which makes it a good choice for athletic or activewear. Polyester can also be printed on using screen printing, although it may require the use of special inks and processes. However, polyester can be less breathable and comfortable than cotton, and it may not create as vibrant a print.
Ultimately, the best option for you will depend on the specific needs of your project and your personal preference. If you want a natural and breathable fabric that creates a high-quality print, cotton may be the better option. However, if you want a synthetic and durable fabric that is resistant to shrinking and fading, polyester may be the better option.