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

The higher the hardness value, the harder the sponge becomes. Generally, a softer sponge allows the ball to sink into the rubber more easily, making it easier to control, more stable, and easier to use.
On the other hand, a harder sponge makes it more difficult for the ball to sink in upon impact. However, if a player can generate a fast enough swing to fully engage the rubber, they can produce more powerful shots.

For more info, please visit Rubber Specifications.