The finer the pitch, the more force you get for moving the slide, but the finer the pitch, the weaker the ball screw because of the small size of the balls and grooves. A finer pitch means you need less motor to move the axis. You probably don't have to worry about the strength of the ball screws though, when you consider the siz of the machine you are putting it on.
A rule of thumb - get the largest diameter that you can put in your slides, that would give you more strength and accuracy for a given pitch.