in-floor heat retrofit; copper or plastic and headroom loss?

I am inching toward replacing my electric base board heating with gas-heated hotwater in-floor heat.
My plumber is suggesting two things that I fear may be misguided (and once I cover the installation with concrete and flooring I don't want to be making a mistake.)

First, my plumber seems to prefer copper pipe tubing instead of plastic. some friends with in-floor heat say that the risk of a copper tubing joint failing is too great and that plastic tubing is a much safer and better option.
does copper tube in this day and age make any sense?

Second, I don't want to lose any headroom in this living area. I understood from my plumber that even with hotwater I only need to lose 1.5" or so once the installation is done.
A friend sugggests that a much deeper concrete pad is required to spread the heat most efficiently.

any comments?

Peter Stock

If you use something like the plywood plus aluminum panels, you only lose maybe 1/2". You attach the panels to the existing floor, snap in the tubing, put down your finished floor and you're done.

Copper in concrete is not a great idea. My unprofessional opinion. Too many joints, plus, copper and concrete are not happy together!