One ball bounces elastically, while the other doesn't bounce at all. The first is made from neoprene rubber, while the other is made of a proprietary material called Norsorex. Norsorex under normal conditions has a large hysteresis and doesn't return to its undeformed shape very quickly. It also has a larger surface friction. Roll both balls down an incline and the Norsorex one rolls slower. This makes it a candidate for racing tires. The properties change as a function of temperature.

Happy and Unhappy Balls