In-slab heating systems work best with porcelain, ceramic and stone tile because they conduct heat efficiently and do not expand or contract with changing temperatures. Floors made of these materials also resist warping and cracking and are available in a range of colors and styles.
Carpets are an option as long as they are not too thick. Installing a densely woven plush carpet with a pad will block the heat from getting into the room. The best choice is a thin pile carpet, like a Berber, along with a thicker pad. Otherwise, a second heat source may be needed to heat the room.