*quote*
Until recently, platinum was considered the best material to use on the top of an electrode because of its durability. However, Iridium is 6 times harder, 8 times stronger, and has a melting point 1200 degrees higher than platinum. Put that into a harsh environment such as an engine piston chamber, and you have a spark plug that can resist wear much better than platinum.
*quote*
For longevity go the iridiums, as long as they are in the correct heat range otherwise they can do more harm then good.
After a while just take the plugs out and give em a gentle scrub with a dry soft bristle brush.