When the car calls for the copper...use the copper. Search the posts here (and archives) for platinum plugs. You will see a plethora of responses supporting this.
I had to get my Cu plugs from NAPA, since all the other auto places (Auto-Zone, PepBoys, etc) only carried the platinum replacements.
"Would you say I had a plethora of pinatas, Pedro?"
"Oh Yes, El Guapo"