Years of seeing piped air in my grandfathers service station as a kid it was all black iron pipe as my wee mind remembers it was about 3/4", drain traps under every vertical rise (it was one of my jobs to drain the water out of the lines, what fun!)... Later when grandpa closed up shop dad went to work for his uncle. Uncle used plastic 1" pipe throughout his new auto repair shop, all went well for the first 16 months he was in business then one of the pipes blew at the fitting and injured a customer who was standing just outside the shop door. They dug 5 pieces of plastic pipe out of the mans chest and arm. Uncles insurance company refused payment due to the installation not being up to the standards recommended by the manufacturer of the compressor being used.
Needless to say, I wouldn't recommend plastic for compressed air, black iron is going to be more expensive but won't be near as apt to being damaged by vibration, impact, or chemicals.