What is the main difference between platinum and iridium spark plugs...is it how long they are good for or is there a difference in performance.

Iridium is stronger and harder than platinum, the melting point is a lot higher, and they last longer without diminishing performance when compared to platinum plugs. They pass spark more easily as well.

Yes, its important to point out the they conduct heat away from the gap /spark area of the plug better because of the thin wire design than platinum which has a "cylinder" like copper plugs

GM actually had found predetonation on boosted 3800s with platinum plugs , hence one of the changes to iridium in 2002. but the platinums were rated for the same mileage, b100k, but an iridium should be able to easily handle the 100k easier than the paltinum. The waste spark also works easier with less resistance too