Hi David,
Piston choices really depend on how the engine will be used.
For longevity, no 2618 or 4032 piston lasts as long as Mahle's proprietary alloy and thats why Porsche used those. Its my
first choice for street cars where engines are expected to last 100K+ miles. You get what you pay for.
For competition engines where piston weight is critical due to operating RPM, a 2618 piston is an excellent choice for its light weight and strength (if its designed properly). CP makes our 2618 pistons and they are all made to our specs for proper fitment to assure ring sealing, less noise and blowby.
We also use custom Mahles made to our specification in large displacement engines.
Piston design is every bit as important as the choice of alloy and they are NOT all created equal. Many people make 2618/4032 pistons and some designs are not well suited for 911 engines.
Naturally, this is a greatly simplified answer to a very complex subject, but I hope this helps a little,