From what I can tell, I just went though this at work... copper is the way to go. I did a walk though of our machine shop and noted that it is a black iron system, but as they develope leack / problems they replace chunks of it with copper. The attached link should offer you a little assurence on rated working pressure for copper as it is well within an air system.
http://www.coppercanada.ca/publicati...ationptb6.html

I think that this is the way to ensuring long trouble free supply of clean air... if you can afford it. There is also a company... TRANS AIR?... that make a powdercoated aluminum piping with compression fittings. $$$$ , belive it or not, copper worked out cheaper for me. Just my 2 cents, but I hope it helps