In HVAC/R work N2 is used because it is 1-inert, 2-cheap, 3-seems to "dry" the system.

CO2 can work, I used to use it, but N2 is drier which tends to give better results. I am sure Argon/CO2 would work for pressure testing, just make sure you evacuate completely then pull a 29.9mmHg vacuum in ambient above 70F (preferably in low/no humidity) for a good long time & you should get good results.