If you live in a hot climate, you may want to avoid synthetics as much as possible. They don't breathe as well as natural fabrics, so they could get warm and make you sweat. Flannel also feels warmer than other toppings, so you may prefer a silk or flat cotton top during the warmest parts of the year.
Recommended materials for warm climates: Flat cotton and Silk Tops. No Synthetics.
Recommended Sampler Sets: Any all-natural option with flat cotton or cotton terry substituted for flannel.