|MadSci Network: Physics|
If the surface of the hill had no friction then both balls should arrive at the same time. Some force of friction is what slows it down. If the heavier object goes down the hill faster, then it must have less friction retarding its motion. The lighter object may have a rougher surface, or some other properties that's making it slower. Friction is what makes the difference. [note added by MadSci Admin: The objects may also have different relative moments of inertia, which depend on the distribution of mass in the objects and affect the rotation of the objects.]
Try the links in the MadSci Library for more information on Physics.