A 10mm pitch screw has to rotate twice as fast as a 20mm pitch for a given linear gantry speed and will suffer more whip as a result. gearing the motor speed down 2:1 on a 20mm screw will give the same motor speed as 1:1 on a 10mm screw but will double the torque driving it. This means the maximum linear force available to push the gantry will be the same in each case.
Less whip but the added complexity of a reduction belt drive.

EDIT: Dean beat me to it!