Ok thought you meant the linear bearings on the rails

For ballscrews my machine is 5mm pitch all round but if I was starting again then 5mm Z and 10 mm for X and Y. This gives better rapids in wood and should be Ok for aluminium but if you drive the X and Y via belt and pulley you can swap out the ratio on X and Y to give 2:1 to give more torque (and less speed) if needed on the aluminium
Not sure about 20 mm pitch - will give very high rapids but you loose some torque and could stall the steppers? Anyone tried this much pitch? May be ok with enough voltage on the steppers . . .