Back a few pages, I read that pex had disadvantages over copper. Not the case at all.
As a installer of radiant heating systems I would like to add a few points.
Pex has come a long way since the 80's. The grades of pex we use today in hydronic heating and potable water supply will both out last any human, or copper line.
Pex is not subject to mineral build up inside of the lines like copper. Over time, this build up causes restriction that decreases performance drastically.
Most good grades of pex are now UV stabilized. The ones that aren't simply need a coat of latex paint to block the UV.
Pex that is made for radiant heating has minimal expansion and contraction.
Most grades of Pex that are made for radiant heating contain a oxygen barrier to stop corrosion in closed loop systems.
And last, and most of all. Please don't make the mistake of installing the lines in your floor with air around them. You will get less that 5% of the heat transfer. Lines MUST be set into aluminum heat plates, or mortared into the grooves with some type of small aggregate cement mix. This is the biggest mistake of DIY hydronic heating. Second biggest mistake is no insulation under the lines to separate the heat zone from the substrate. Steal bus floor in your case.
Take care, Nat