I'm just way too anal to put cheap parts on my car I guess, I have the Iridium plugs on my N/A, stock headed motor, and obviously they don't really offer any noticeable performance advantage other than you never really have to change them.
I think I just got suckered into that cool "hot red pepper" advertisement scheme.
Everyone should know at this point that copper is the best conductor of heat/electricity, therefore redering it the most beneficial choice in overall performance. Platinum is not as good of a conductor. However, you won't have to change platinum plugs near as much as you would with copper.
I run NGK coppers in my WRX, and Autolite coppers in my Mustang.
Stock WRX plugs are platinum, so I actually noticed BETTER performance when switching to the coppers. Night and day difference...go with coppers (for everything).