Rule #1
If it ain't broke, don't fix it

Seriously, just stick with the Plat's

There was a discussion about Iridiums on Maxima.org a while back. I read
up on Iridiums and found some interesting facts;

first, Iridiums don't last as long as Platinums. The Plats will go 60K,
but the Iridiums will go 30K. Funny how they tout the hardness of
Iridium being better becasue the metal is harder, yet they don't last.
Second, copper plugs are still the best type of plug for producing
spark, but those things don't last at all, these modern distributorless
ignition systems will wear them out right quick. If you go to the web
sites of company's like Denso and NGK who sell iridiums, sometimes
you'll even see charts where they compare metal's hardness and
electrical properties. Suspiciously absent is copper in these comparisons.

 If you don't want to change your plugs often, just stick with the
plats. IF you don't mind changing your plugs ore often, then you might
as well get copper because they're cheap and have the best electrical
properties for a spark plug. If anything, an iridium plug may make your
car have cleaner emissions, but there won't be a night and day
difference. I think a vehicle has to be designed from the ground up to
take advantage of what Iridium has to offer. The Maxima was designed
with Plats in mind, so they're hard to beat.

CD

What about silver ?

I went through this a while back, and decided Iridium was an expensive
gimmick with no tangible benefits.  Copper is cheap and will last
30,000 miles.  Good if you have a supercharger or such.  Otherwise,
Platinum is less hassle and is very effective.

Hardness has nothing to do with how fast metal will vaporize at high
temps.

I've read (here) that Pt plugs can be found on ebay cheap.  I got mine
from Garrands Nissan in New York for a little over $8 each by mail.

JM