The advantage of copper is it is proven. That is it's biggest selling point, you know it works and it works a long time.

Pex isn't necessarily new, but it hasn't been around as long as copper.  The advantages are a lot faster installation, easier to run a continuous piece with no hidden joints, and lower cost.  My folks house has silver soldered hard copper.  I spent years installing soft copper with flared fittings in the propane business.  My son-in-law put pex in his garage floor before he poured it last summer.  The pex is far and away the easiest material to work with, and I believe least costly.

I don't know, but I would guess the pex expands and contracts a little faster than pipe or tubing.  I remember looking at a steam pipe running above ground between two buildings and asking why it had those unnecessary loops in it.  I was told it was because as it heated and cooled the pipe changed length.  Don't see why pex would be any different.  As long as the installer follows instructions, it shouldn't be a problem.

Something I just thought of.  The folks' house has hot water heat and there is always popping noises as it heats and cools, and you hear water flowing at certain times when it is working.  Would pex do that as well, or would the more flexible material be quieter?

Which would I choose? I have an old house plumbed with a combination of steel pipe, copper tubing, PVC, and CPVC.  Pex sounds pretty good to me when I go to working on this mess.

You said "One contractor will not use pex - copper only"  I would ask him why?  Is it because copper is better, he is unfamiliar with pex, or is he just stubborn and refuses to consider something new?  Because pex is pretty much proven technology.  If it is "Well, I just don't like it" then that opens up his entire program to scrutiny.