Matt, I don't know the size of your compressor, but copper has been known to burst when used for air delivery. If you have a relatively small unit it should be okay, if not, I'd probably go with the gas fittings.
Have to agree here.. it work hardens and the cycling from the fill/drain will eventually kill it! A building inspector went nuts on us for using PVC in our shop in Portland. Said it should only be black pipe.
Use black pipe. 1/2" should be fine. It's actually cheaper than copper right now and easier to work with and modify later.
Thanks for the concern. I double checked with local building codes and OSHA to see if there was anything prohibiting the use of copper tube. Local code states that it has to be ASTM B88 tube, which types K, L and M match, and that system pressure will dictate what wall thickness needs to be used. I couldn't find any reports on copper regularly failing from compressed air. My compressor is 135 psi, 10 cfm: it's a decently sized home garage compressor, but definitely "relatively small" compared to industrial units. The material's rated pressure (850psi) is well above compressed air pressure. The soldered joint is the lowest rated part of the system at 515psi. I'll be isolating the copper lines from the compressor with a flexible air line to prevent vibration from fatiguing the material. Practically, the shops at work all have copper air delivery systems, and I'd like to make mine similar to that for the sake of commonality and comparison.
Using PVC hadn't crossed my mind and never will.
Black pipe was $0.50 cheaper for a 10ft stick, but the fittings were much more expensive, as well as buying smaller cut-to-length threaded pieces. A pipe threader would've been more than the tools to cut, prep and solder copper, too.
With that said, I went to get my copper pipe last night--the 10 ft sticks were sticking out of my sunroof. It was chilly!