A copper core plug is better in a performance standpoint for a supercharged engine that is modified. The reason behind that is the Iridium while lasting a much longer time have a pinpoint tip. This tip of the harder iridium heats up more than a standard copper plug tip. That extra heat can cause predetonation in higher than stock applications. The down side of the copper is, you need to change them every 20-30K miles at most. Therefore you spend about the same or more over the useful life of the Iridium plug.
If I were running a stock car, Irids would be my choice due to reliability and useful lifespan. In my modded situations, I run the Autolites instead of NGK due to a bad experience with screw on NGK plug tips. Same price, no KR from loosening tips....gimme Autolites.