copper vs pvc compressor lines?

NEVER PLASTIC OF ANY KIND.
1) An air molecule is smaller than a water molecule that is why plastic explodes with air and not with water.
2) As for "schedule" basically the higher the number the thicker the wall. This applies to Most piping, copper uses K, L & M. Schedule 10 pipe will have a thinner wall than schedule 40, schedule 40 thinner than 80 so on and so on.
3) Most piping is measured ID (inside diameter)or IPS (inside pipe size).
4) As for size it depends on the linear footage of your mains and how many drop legs you require. For example a 30x50 shop with a air line on three sides (doors on forth) I would use 1-1/2" main and 3/4" drop legs (at 10' per) assuming there are three per wall. If your drop legs are longer than 10' bump it up to 1".
5) "40" black steel piping is all I use and malleable iron fitting NOT cast iron!

My 2 Cents.
:burnout: