I spoke to my mechanic about this and he is a good friend of mine since I've been going to him for close to 3 years and so has the rest of my family so I know that he isn't bullshiting me..
here is what he said..
The normal spark plugs that you can get from NGK or Champion with the normal copper tips will FLOW (if that's the word to use here) more electricity than the platinum and iridium ones.
He showed me some stats from tests that were done at some industry show that he attended.
The only good thing about the platinum or iridium plugs is that you don't need to change them for something like 50,000 kms or more.
The standard copper plugs are supposed to be good for 20,000kms.
And if you're car runs rich (lets face it, most skylines do) then you are going to have to change any plugs you have more frequently.
Don't know about you guys but I am not about to spend a shit load on spark plugs that are simply going to last Longer. I'd rather change them every 20,000kms and have brand new ones in there more frequently, that also FLOW better.
Also, he recommends that cleaning them to get more life out of them is a waste of time.