What size compressor are you running? May make a difference as to what size line i'd run. 10-15 cfm, should get by with 1/2" line, 15-30, I'd run 3/4.
Back to the "plumbing" guy at Lowes, I think you stumbled on someone who just knows you'll be back for some cutting and threading of black pipe (which they do free of charge). With PEX, he sends you out the door and doesn't have to get his hands dirty.
The benefit to PEX is that it's easier to install with typical tools. Copper you'll need to cut and solder. Black steel you may have to cut and thread. Unfortunate to the "plastics", they are quick to assume the temperature of the incoming air. So after a couple cycles of your compressor, the tubing is up to temperature and does little to "precool" the air prior to reaching your moisture trap. This lends the PEX as a better fit for an install that uses a cooler or dryer device to condense the moisture.
For the metal pipes, and likely something you'll see in the above TP tools link, most compressor/airline manufacturers such as TP tools recommends at least 25 to 50' of pipe prior to your water trap so that the metal pipe will cool/condense any water vapor out so the water trap can do it's job. If you've ever had water dripping out of your air tools even if using a water trap, it's because the water has not condensed and passes through the water trap as a vapor, and is condensing on the other side. If you live in a dryer climate, you may get by with closer to the 25' of metal pipe, more humid may require closer to the 50'. So IMO the metal pipe would be more suitable if you had no means of condensing the compressed air prior to a regulator/water trap. (cooler, refrigerant dryer)
And as an example of what NOT to use, here's an article on PVC pipe that exploded in a factory, sending many of the employees to the hospital with plastic shards imbedded....
http://www.wave3.com/story/17113807/explosion-at-fairdale-factory-injuries-reported I guess there is a reason OSHA has the stuff outlawed for compressed air..
