PW Bros, Please note below my suggestion;
1- The best conductor is copper: copper plugs last less but gives best performance, no over heat-up on extensive ridding.
2- Iridium semi conductor between copper and platinum, last longer than copper but not the best performance.
3- Not recommended for bikes less than 250cc.
4- have you ever seen coil winding with some other metal than copper ? No!
5- from the coil through to plug current should run/flow without any break, thus iridium or platinum type of plugs arrests flow of current.