I also use the air cylinder setup, check it out at http://www.poly-cor.com
look at the CNC page
Actually the belt is controlled by probably a stepper motor which has holding torque and since there is really no weight involved in the head it shouldn't be a problem. I have seen springs used to control tension on many foam cutters.. The air cylinder idea is intriguing must be a low pressure cylinder to prevent breaking the wire at high temps.Since belt has no holding torque, I was thinking is have to be screw.
Yes it does....about the only difference is the control and positioning is more accurate with a servo. And more expensive.... maybe overkill for a foamy.