from what i had read iridium can last longer than platinum. performance wise, iridium should be better than platinum as well since the centre electrode is finer.

personally i never tried platinum. i used hks iridium, so far so good, never had any plug issue even on high rev 9k rpm. the only downside i can think of is the price tag...

agreed with sleepwalker, u can't expect hp gain just from iridium spark plug. if u have hp gain by just adding iridium plug on stock engine, then ur previous plugs must had fouled out.