Epoxy putties are two-part compounds which, once mixed, will permanently cure at room temperature into a tough, rigid, plastic-y material. There are many varieties of epoxy putties, making it extremely versatile stuff!
Clays made with a base of oil or wax (or a combination of both) are a type of sculpting medium that will never cure or harden completely. This makes them ideal for sculpting a large project that you may need to work on over a period of days or weeks. When the sculpture is complete, you make a mold from it, and then cast it to get your final piece.