For plastic and ali I would go for the 5mm pitch screw to keep the torque advantage. You can always gear up if using belts. Diameter is dependant on length/speed but 16mm should be ok.

Ross obviously I got this one wrong, still learning myself & haven't started machining yet other than a few test pieces. Reason I had said 10mm pitch screws was because I was under the impression that you had to use fast speed & feed rates for cutting plastics. Have I got that wrong or would you just gear up 2:1 for that.