Most plumbers consider CPVC to be the better choice, since it is a time-tested material and PEX is newer.
However, PEX has the advantage of being able to bend around corners and uses barb-type crimp fittings that are easier to install.
The process of removing pipes from behind walls and under floors can seem daunting, and you may be tempted to leave some of them in place.
This undermines the retrofit, because the pipes you leave will deteriorate and cause problems long before the replacements will.
It was especially popular with mobile home manufacturers, and is usually gray, with copper-color crimp fittings at pipe connections.Always use two wrenches on joints that are difficult to unscrew.