Recycled: About half of the copper used in the United States—which would include plumbing pipe and fittings—is created from recycled copper, according to the copper industry’s trade group, Copper Development Association, Inc. (CDA).
More rigid: PEX’s flexibility is great when you want to go around corners but bad when you need to stub out to a toilet or sink. For that, you need to either use a copper stub-out or buy special PEX fittings for his purpose.
More heat resistant: PEX is heat resistant and can even be used for under-floor radiant heating. But to connect to high-heat services, like your water heater, you need to make that final run with copper or special stainless steel braided connectors.
Will not give off toxic fumes: As CDA points out, PEX is plastic and will melt and emanate toxic fumes in the event of a fire. Copper has a far higher melt point and does not give off toxic fumes.
Cheaper fittings: If you do decide to use copper fittings, they are cheaper than push-fit fittings.
Value of copper: The copper pipe that results from a demolition project can be sold because the material is typically valuable enough. This cannot be said for most building materials stripped from home and certainly not for PEX pipe.