I've been filling my PCP's with nitrogen since I got my first Talon about 2 years ago. Like someone said, no appreciable difference between air and N2. On the other hand, I've read claims of people getting extreme velocity by using helium. It seems counter intuitive, but apparently the lighter the gas, the faster the pellet. Main problem with helium is leakage, the gas molecule is so small that it leaks past the valves. Based on that, I'd think the argon would slow things down, but it would be interesting to find out for sure. Try both and post some results. Later.
Dave