Helium is a low density gas that has a much higher flow rate through chocked high delt-P orifices.
Bottom line, air or N2 will see sonic conditions in the wheel guns at much lower velocities that will helium. Advantage He.
To address the issue of the scarcity of helium .... another small oily coloured fish. Prior to about 2003, the US Government mandated that Helium was a Stratiegic Material. A legacey from the Civil War and the WW-II
http://portal.acs.org/portal/acs/corg/c ... 443bb33e8b
This Stratiegic material designation effectively subsidized the cost of production so it was cheap and readily available. Once the subsidy was removed (Economics 101 for those Greek Scholars amongst us) the price went up, scarcity issues surfaced and panic set in.
Helium is naturally occuring and is produced in measurable quantities in natural gas. Virtually all gas contains some amount. The expensive part is the separation process, not the scarcity of the material itself.
On the issue of moisture in compressed air or in N2, same issue with the deep-water reptiles mentioned earlier.
If you run air at 6 to 10 Bar out of an air compressor, you will condense out moisture. This will wind up in the air guns and cause corrosion. N2 on the other hand comes from compressed gas cylinders that are dry simply because at 2000 psi in the tank you can not tolerate more than a miniscule amount of moisture. The gas is dried (PSA or TSA process) to about a -70 deg C dew point (at atmospheric pres) or if you are lucky enough to get your supplies from Edmonton, dried to -40 deg C at 2000 psi, even dryer.
Bottom line, if you had compressed air from cylinders at the same pressure as the N2, the moisture content would be the same.
The teams use cylinders because they are convenient, dry, easy to get and pretty darn cheap. And they come delivered at 100+ Bar. No need to fuss with powered compressors and likely small annoying gas dryers.
Personal motto... "Were it not for the bad.... I would have no luck at all."