You would run them both through an HP reg, you will usually find CO2 to produce a higher speed at the same pressure with different valves having higher differences.
This is because CO2 has a low compressibility factor(iirc ~0.4 compared to 1.1 for HPA at 1000psi). Meaning it is denser at high pressure and as the pressure goes down it expands faster. It doesn't necessarily mean it is more efficient by weight(it isn't) but combined with condensation it is obviously more efficient by space. The sound thing is related to the speed of sound in different gasses