The most defining thing that separates modern from classical is that the latter is strongly representational - it shows people and horses and lions, etc. as they are supposed to look, while the former avoids that and goes for shapes/forms and spacial relationships. In fact the dividing line between old and new is often put at Rodin's sculpture of Balzac as discussed here http://www.bluffton.edu/~sullivanm/rodin/Balzac.html where it was claimed also that there was no person inside the drapery for which Rodin said - "If I were to strike the plaster model with a hammer, you would find the nude statue inside" and in fact copies of the nude model exist.
Since modern sculpture includes thousands of examples in marble and bronze that are very abstract it is unrealistic to refer to contemporary materials although a lot of modern sculpture does use steel, plastic, aluminum, and found objects which are not used in classical work.
