First lets consider why someone may want an air saw if you already own a sawzall or otherwise known as a reciprocating saw that plugs into the wall or is battery operated.
Size and accessibility! An air saw is magnitudes smaller:
The problem is that air saws only have enough power to cut through sheet metal thickness.
What if they could be more powerful?
Well thats exactly the difference between Gear driven and Piston driven air saws.
Gear driven air saws have much higher torque, and reciprocate slower; nearly half as slow. Piston style air saws reciprocate around 10k RPM, where gear driven reciprocate around 5.4k RPM.
Below is a cut away of what a piston style power system looks like. I couldn’t find a cut away of a air saw specifically, but they are very similar. The main principle of operation is the same.
Force is equal to mass*acceleration. The principle behind a piston driven system is to accelerate the mass (blade) very quickly; It however has little ability to sustain that speed under load (torque).
The gear driven system uses a air motor attached to a gearing system, to allow a mechanical advantage by converting acceleration for sustained torque. The end result is more power, no stalling and less burned out blades.
The trade off? Cost; gear driven air saws are a premium item and cost twice as much as their piston counterparts.
But when space is tight and the job needs to get done, I think its worth the extra $100.