Platinum plugs are for fuel economy. Iridiums are for better performance.
Interestingly, the best plugs are copper if u can change plugs regularly. Copper is the best conductor, but the issue is that copper plugs have a short life. 10k and you're done with them.
Platinum are the other extreme. They last above 100k kms and are fast becoming standard in many cars internationally. However, performance is nowhere near that of a copper plug.
Iridiums are good because they give performance much better than platinum but still nowhere near copper, and last around 80k to 100k kms.
This is where denso ik20 is something of a mismatch. The denso has a 0.4mm gap and life much lesser than a normal iridium plug. It sacrifices life for performance closer to a copper, for far longer than an ordinary copper but far lesser than ordinary iridums.
If u can manage, copper plugs changed regularly are the best option. Otherwise, ik20 is the best of both worlds.
Sent from my GT-I9300