18 and 20 tooth pulleys

Which one is best for 500mm x-carve and why?
I am upgrading to NEMA 23s 140oz hold and see many choices.
Advice?

The ShapeOko wiki has some information and theory:
http://www.shapeoko.com/wiki/index.php/Belts_and_Pulleys

From what I read, the more coarse (smaller) the tooth is, the more it can handle more torque, at a cost of precision.
I guess you can stick with 20, and that way your math / calculations wont change much. Plus 20 is what the majority of X-Carve / ShapeOko users are running.