These debates are often filled with myths and wive tales. I actually called ngk and talked to them about the differences. I agree that if you are using a copper plug that it has to be a non resistant plug. Platinum plugs should be non resistant to. Iridium on the other hand have to resistor plugs. The reduced tip size brings the arc voltage down to a much lower kv. The spark jumps the gap easier. I run iridium in my m103 and m104 and m112. I have had no problems with them. The problem with adding resistance with a copper plug or platinum is that you are creating in essence a harder path to the ground. With platinum plugs you could gap down and some platinum have a smaller tip and lower arc voltage also. So regardless of what you are using, copper, platinum, iridium, they all need to be correct for the application.
My preference at this point is the iridium plug. They last forever and the idle is better. The small tip platinum plugs wear down over time, but work well.