I'd recommend mainly sticking with high-quality instead of high performance. You will want the Japanese made plugs. Stay away from Bocsh plugs. They wear out too quick. Denso makes the best for Mitsubishis. They list them as NGK spark plugs.

As far as copper verses platinum plugs, there's very little difference. Copper is less expensive. But if you get copper, you have to gap them properly. Platnium is more expensive, but you never need to gap it. Though the gap on the platnium plugs is set for many different vehicles; it cannot be gapped specfically for your car. And you can never gap platnium plugs yourself.

Feel free to ask me any more questions.