basic rule of thumb is Acme is usually imperial, trapezoid is usually metric so you get 1/2" acme versus 12mm trapezoid, also often know as power screw over here.

Acme is usually a cut thread and trapezoid is usually a rolled thread.

Another thing to bear in mind is that 10 tpi two start is worked out as 5 tpi, the greater the tpi the more torque you need to move a carriage.

Good website here.

http://www.kingston-engineering.co.uk/exstock.htm

John S.