In recent years, linen has become more popular for all kinds of knitwear and some offer linen for polo shirts. With its crisp look and sophisticated wrinkles, it certainly adds another dimension but it is also much rougher than cotton. As such it is only recommended in blends if you want the crinkly linen look.
With moisture wicking abilities, breathability, and decent durability, cotton shirts are the most common polo shirts found today. Now, not all cotton is alike, and cheaper cotton polos use short staple cotton that will cause pilling and faded colors after a few washes. Of course, a longer staple cotton will last longer and likely feel better on your skin, however, it will also fade in color eventually especially with darker colors. That aside, the quality of a polo shirt does not just depend on the materials but also the knit.