For normal operating conditions I recomend OEM spec Platinum or Copper plugs. Replace them more often instead of splurging for more expensive, over rated iridium versions.
Here is some info:
Simply put, in high performance N/A engines there is no need for iridium (temp.) and platinum does afford some benefits over copper in a stressed engine. In FI engines, iridium may offer some advantages but using platinum does not hurt performance. You can use platinum coated copper electrode plugs which give you the heat advantage of copper and the arc-voltage advantage of platinum...just to throw more confusion into the mix.
So, platinum provides a smaller sharper electrode to allow a larger arc-over-voltage and will burn off before allowing an engine to detonate. Yes it is more expensive but it lasts longer.
Basically the smaller the electrode (has less Capacitance for those into electrical stuff) then less voltage it needs to fire. The bigger the electrode the more current it can flow (good in cars with heaps of power available, huge sparks).
http://www.team-integra.net/sections...?ArticleID=419