I would have gone with copper plugs since copper is one of the best conductors available for spark plugs and it's relatively inexpensive, but I don't want to go through the hassle of changing them every other oil change or so (~10-15k intervals). Nickel and platinum plugs are ok conductors, but they're mostly used for their extended life span over copper. Iridium is the best of both worlds - good spark and long life. Even though it isn't the best conductor (which is why tuners may sometimes advise against them - if you're that concerned about tuning your car, you should be using copper plugs anyway), but it's extremely hard and it will last the longest of any spark plug material (some are rated up to 100k). Subaru recommends changing the plugs every 60k, so NGK Iridiums fit the bill with plenty of room to spare.