If your house was built before about 1970 and still has its original supply lines, they are probably galvanized steel. Galvanized corrodes from the inside, and the failure mode is gradual: pressure upstairs drops first, hot water turns rusty after a vacation, and eventually a pinhole shows up in a wall. Once a house reaches that stage, patching individual runs is throwing money at a system that is uniformly at the end of its life.
Copper is the traditional replacement. It is rigid, it tolerates heat without any question, it is unaffected by sunlight, and a properly soldered copper system is genuinely a lifetime install. It costs more in both material and labor, because every direction change is a fitting and every fitting is soldered.
PEX is flexible tubing run in long continuous lengths, often from a central manifold with a dedicated line to each fixture. Fewer fittings means fewer potential leak points inside walls, it installs faster with far less demolition because it can be fished through framing, and it handles our region's aggressive water chemistry without the pinhole corrosion that occasionally affects copper.
In practice we specify copper where lines are exposed — garages, exterior walls, mechanical rooms, anywhere UV or physical damage is a factor — and PEX for the concealed distribution inside walls, attics and crawlspaces. Most of the repipes we do in the Inland Empire end up being a sensible mix of both rather than a purist choice.
The thing that actually determines how a repipe feels to live through is not the material. It is the route. Ask to see the proposed run and every wall opening marked before anyone cuts, ask how long the house is off water each day, and ask in writing who patches the drywall afterward. Those three answers separate a clean repipe from a memorable one.