The quality of a table tennis ball is determined by 4 criteria
(according to the rules of the ITTF International Table Tennis
Federation)
Weight: A lighter ball is quicker than a heavier ball
Hardness: The harder the ball, the livelier and more solid it is. Also a harder ball will retain its roundness for longer
Roundness: Even
the smallest dent in a ball can affect the direction that it will
bounce. So the harder the ball the less likely it is to dent and the
longer it will retain its roundness
Rebound: A ball with a good rebounds means the ball will bounce at a consistent height every time.
As good as any ball is the quality will decrease the more you use it, so it is important to change the ball on a regular basis.