From my search:
Copper spark plugs are generally considered to have the best performance of any spark plug type. This is potentially different from what advertising companies suggest, but the other metals are, unfortunately, not as conductive in general as copper is. Platinum and iridium plugs are more likely to overheat, which causes damage to the plug components and can compromise the delivery of the spark to the engine block.
If Iridium or Platinum is superior to copper then why is it that race cars (motorcycles, boats, etc.) all use copper plugs? This industry is all about performance. If the alternative to copper gave them even the tiniest of advantages they would be in use.