PE-RT Type I is the default for residential floor heating because it is easy to work: bends by hand, welds with standard PE fittings, no cross-linking to worry about. If your system runs at 50–60 °C supply, it is entirely adequate and it is the cheapest of the three.
PE-RT Type II and PEX-a move you up the temperature scale — 90 °C continuous, 110 °C peak — which matters if you connect to a district heating substation or a solid-fuel boiler that can run hot. PEX-a also has the best shape memory: kink it, warm it, and it comes back.
What nobody tells you: the pipe is rarely the failure. Bad manifolds, missing oxygen barriers on open systems, and loops that were pressure-tested with air and never re-tested with water cause more callbacks than the pipe material ever does. Spend the difference on a better manifold.
