I feel the accuracy of a lead screw, depending on pitch, is superior to rack and spur gear,IMO. The thing is, the accuracy obtainable by rack and spur gear is greater than the accuracy of the plasma itself.
I started out with roller chain and sprocket, and decided to go spur gear later on. I am quite happy with the accuracy I get, whether I am using oxyfuel or plasma.
I am using 20 pitch, 20 degree pressure angle.
Another side of it, is that you will get faster speeds with spur gear and rack, over most lead screws. You should gear down the motors to obtain your torque, whether with gearboxes or timing belts, etc. and then drive your spur gear from there.
Plasma needs a moderate amount of torque and fairly high (200-350 IPM) speeds, at least for thinner materials. IMO, leadscrews wouldn't work to well at those speeds.