Air is much better. CO2 goes through massive temperature fluctuations which will make your velocity spike an drop while playing. Since most markers are as accurate as any other, depending on barrels, then velocity consistency is your best measure of accuracy and HPA is more accurate. CO2 will also destroy the internals of electro markers due to the very low temps, and also causes excessive wear on orings in general for the same reason. With steel HPA bottles going as cheap as they do, there's no real reason to go with CO2 unless it's the ONLY propellant you have access to....which isn't the case. HPA all the way dude