As someone who has learned from spending all day long in a costume at a Renaissance Festival, I can tell you that you do not want polyester, a poly blend, or any other man-made material (such as nylon, spandex, or lycra). They don't allow for breathing very well, so you will sweat more. Cottons, light wools, and other natural fabrics like muslin are ideally suited for long-term costume wear, as they breathe a lot better.
The number 1 problem with cotton is you have to iron it. Or it looks like hell. Wools can be itchy and are dry clean ONLY. Muslin also has the rep for being itchy. I rather be a little warm, than itchy.
I normally wear a cotton undershirt so I never have problems. Also I live in the Northwest and wear 90% of my costumes in the Fall, Winter and early Spring where if it gets above 65 -70 its weird.
Buy your fabric for the look and feel, worry about what its made out of after you pick it out. Just remember if its a little stretchy its going to be harder to work it. And if its cotton you are going to have to iron it ALL THE TIME to keep it looking nice.
