Since you're asking this question in the Physics category, I'm going to assume you're not asking which surface is easier to clean or looks better in a kitchen. I'll answer your question from an engineering standpoint.

Microwave ovens require that the microwave energy be confined within the cavity. If the energy leaked out in significant quantities, it could disrupt wireless communications devices like cordless phones or wireless networks, possibly start a fire by heating nearby materials, or even injure a person in proximity. Furthermore, it wouldn't cook your food nearly as well, since much of the energy would leak away, rather than being absorbed in the food. For these reasons, the microwave cavity is always surrounded by a metal enclosure. Whether solid or perforated, the metal is opaque to the electromagnetic waves, and therefore keeps them confined within the cavity until the energy is absorbed by the food. (The perforated screen in the door, however, is NOT opaque to visible light, which has a much, much smaller wavelength than the microwave energy used to cook the food. That allows you to see inside while the food is cooking without allowing any of the microwave energy to escape).

Hence, the answer to your question is stainless steel.

Of course, there's no reason the microwave cavity couldn't be enclosed in ceramic-coated stainless steel (or any other metal). That would accomplish the same thing, but would probably provide a more attractive, resilient surface for cooking. I'm not really sure whether that falls under your definition of ceramic.