tl:dr Cotton is good, a little poly never killed anyone, and thread counts don't tell the whole story
Ever since George Constanza convinced Yankees manager Buck Showalter to change the team uniforms from polyester to cotton, fabric makeup has been of great concern to the American public. Cotton's great and poly is evil, right? Not so fast, hot shot. There are a lot of misconceptions about shirt fabrics, what's good, and what's not so good. The short version: it all depends on your priorities.
Step 1: Fiber
Cotton: Humans have been making clothes with cotton for ages. It's breathable, soft, and easy to work with. Thanks to cheesy 90s commercials by the cotton industry (“The touch, the feel, of cotton - the fabric of our lives"), along with terms like “Egyptian Cotton", cotton is seen as the supreme fiber of the land. Many things we hear about cotton are true: on a hot summers' day, it will provide the most airflow for your body. Depending on things like thread count (which we'll touch on later) and weave (TMI for our purposes, but Cotton Inc. has a good overview if you are so inclined), it can be as soft as a baby's butt.
Cotton also has its flawbacks. By the end of a workday, or after throwing a shirt haphazardly into a suitcase, cotton can, unlike the Real Housewives of Orange County, show some wrinkles. It also can take a while to dry (REMINDER: DON'T TUMBLE-DRY YOUR SHIRTS). And on that note, they can shrink. Good news for you, we allow for slight shrinkage of all our shirts.
Polyester: Given a bad name by the leisure suit, polyester is science. And we like science, right? Poly, like your 5th grade PE teacher who just might lose it at any point, needs to be treated with respect and understanding. 100% poly does not make for a great shirt. It can have a certain sheen that we're just not that into for a button-up.
But, when you blend polyester into cotton, good things happen. The shirt will wrinkle less, it will shrink less, and hell, it becomes more affordable. Throw a cotton-poly blend into a suitcase without a fear of taking it out looking like a crumpled up newspaper. Sure, it's not going to be baby-butt soft like cotton, but life is all about trade-offs.
Step 2: Thread count
Another area rife with confusion and misconceptions. We all strive to sleep on 1200 count Egyptian cotton sheets, right? Bigger is better and all that? Hold your horses. Let's understand thread count first.
Fabrics are made when you weave together vertical and horizontal strands of yarn. Thread count is the total number of threads contained within a square inch: so if you have 30 threads in either direction, we're talking a thread count of 60.
Higher count therefore means that the individual threads are thinner and more packed together. Holding all else constant, the higher thread count means a softer feel thanks to these thinner strands. Holding all else constant, I said. Good, you were paying attention.
Other factors that contribute to baby-butt softness include the cotton's origin, the type of weave, brand of moisturizer, and its finishing. The fashion (and bedding) industries have taken a shortcut into tricking you to pay more for higher thread counts. Sometimes that higher thread count is going to feel thick and suffocating because the threads are not fine enough and are just packed in to make the count bigger. Screw you, corporate America, and your bigger is better mind games.
Step 3: Ply
There's one ply, and there's two ply. No, not like toilet paper: ply here means how many threads are weaved together in either direction. Two threads weaved together is 2-ply: pretty straightforward. One ply fabrics tend to be lighter weight, and a little more breathable. Two ply fabrics are a bit stronger. That's all. So don't go weightlifting in your single ply shirts, even if they're immensely breathable.