balls. it can be a good idea to have good quality balls since the pitching machine could destroy the balls over time by possible tearing the stitches of the balls. you could also research so that you know which machines use what type of balls (so you could see which could be used to practice for the match and which ones could be used only for training), plus what the maximum amount balls that the machine can hold.
another aspect to look at when thinking about buying a pitching machine is: what else it could offer you. yes, hitting the ball is an important part of baseball and softball but so 