There isn't a lot of choise actualy if you're building a machine that will be competitive.

The rules state that the maximum pneumatic pressure in the robot, anywere, is 1000psi/68bar max.

A typical heavyweight will use 1 2kg aluminium fire-extinguisers as main storage bottle.

That bottle contains 1024L of CO2 gas(if expanded to 1 bar)

The same size of bottle filled with 68 bar of compressed air will contain 200 liters of air.

Pity, as HPA bottles filled to 4500psi will contain 90% of the gas as a CO2 filled bottle.
And will avoid the problems associated with CO2.