SHould I use a platinum or iridium spark plug with my engine?

I got curious on this and found one comment about these.   I don't know if he is right or wrong.

   " The 41-101 is the iridium version of the 41-993 which is the platinum version. Same temp range.
     When ordering through Volvo they used to send the 41-993 for their plug kit number 21467472 but
      now another plug (maybe 41-101)"

The rule of thumb is that platinum spark plugs are more durable than copper based plug tip and that
they run a little hotter.  (just a note - a single platinum plug is a copper spark tip with a platinum disc
welded to the center electrode while a double platinum plug has a platinum disc on both the center
and side electrodes).

Iridium is supposed to be six times harder (whatever that really means) and eight times stronger (longevity?)
than platinum with a 700 degree higher melting point.   They have very fine electrodes and report that they
can last up to 25 % longer than platinum.   The fine wire center is designed to conduct electrical energy
better.