Platinum plugs are used as oem because they last longer than copper plugs. This gives them better long term emissions, and allows for a much greater period between plug replacement. You should replace copper plugs at least every two years, while platinums are usually rated for at least 60K miles.
As far as performance is concerned, copper is a superior conductor and will yield better performance (however slight it may be). More important than the electrode metal is the amount of gap you can/should use in your application.
i changed my plugs and COPs at ~85k. i used the motorcraft platnium plug and theyre working great so far
for all intents, my car is stock. theres no gain in using copper plugs in my car. especially since platnium reduces the risk of spark plug blow out since i dont have to change them as often.
you dont need to worry about blow outs since you have the longer thread head design.
I tried the Bosch regular platinum and they seemed fine at first, but after
about 10k miles my car started getting slow to start, I had a thread asking
why my car was acting like that, but didnt get an answer. Any way I decided
to go back to stock Motocraft platinum, and Boom the problem was instantly
solved. Never again will I run Bosch in a Mustang.
Put a magnet on the tip of a Bosch plug and then a Motorcraft. The magnet
will barely stick to the Bosch but will grab the Motorcraft like crazy. They use
some kind of F__'ed up alloy.
I'm going to go with the Autolites. I have always had good luck with them in all of my Fords. I had 3 COP's go bad 2 weeks ago. I only have 59,xxx miles on the car. I have had it for a year and a half (bought with 46,000 miles). The previous owner used and abused the dog crap out of this car before I got a hold of it. It's throwing codes for the COP and I pulled the plugs and found a broken one. Other than this little hiccup, the car has been a dream to drive and this is the FIRST ever problem I have had on it, other than normal wear and tear.