Definitely stock plugs, they will be the cheapest and the most effective. There is no magic in "performance" plugs, and any gains you get will just be a placebo (which is rarely enough to offset the hole in your wallet). If the magic plugs really gave you +10 Power then you can bet your left nut (or lady equivalent) that they would be standard equipment. Manufacturers try damn hard to milk every last bit of efficiency out of their engines, mostly in the name of fuel economy, and if a simple plug could do it, they would. Iridium/platinum plugs are increasingly becoming standard equipment, along with synthetic oils, not because they give any performance increase, but because they allow longer service intervals (Iridium plugs can last well over 100,000kms before they wear out, compared to about 30,000kms for standard plugs). If you want to buy Super4's or Iridium IX's at $25 a pop, be my guest, you won't break your engine or anything, but know you could have gotten exactly the same effect for about 1/4 of the price by getting genuine. Also your mechanic is less likely to snigger at you when you take your car in for it's next service.