Hardness mainly depends on personal preference and usage environment.
In general, harder pool tables are better for players who need more power on their shots, while softer pool tables are better for players who need more control.
Also, harder pool tables offer better bounce and control, but require more maintenance and upkeep. At the same time, it is recommended that when choosing a pool table, you should pay attention to the size of the room and whether carpeting is required to protect the ball and floor.