Amazing the amount of information and opinion on the subject of compressed air piping.
I came across this article whitch is a bit technical but seems to explore most all of the possible options.
The new flexible aluminum piping looks real slick but the fittings are very expensive and it seems the mfg. only warrantys it for one year.
PVC is out.
Black iron and galvanized are real good however corrosion and flaking galv. are issues with that.
It seems that PEX is not really suited for this application due to sagging on straight runs and not being rigid for outlets and accessories.
Can't afford stainless.
Right now it looks like copper is the most attractive option.