What is the difference in performance according to the sponge hardness of the rubber?

The higher the value of sponge hardness, the harder it is. If the sponge is hard, the ball does not dig into the sponge when hitting the ball, the ball is faster due to short dwell time, and the weight is heavier in general.
On the other hand, if the sponge is soft, it is easy for the ball to sink into the sponge when hitting the ball, easy to control (stable and easy to use), and light in weight.