copper = better conductivity than iridium and platinum.
Honda went with platinum because they have a longer service life. Changing plugs is really easy on all Honda motors anyways, you might as well pay less for a better product. Platinum and iridium is for the people who are intimidated by doing regular mantianence.
Iridium's whole selling point is that they are strong enough to widthstand more cycles of high heat/compression so they can make the tip sharper, and therefore a more predictable and precise spark. However, spark precision really isnt a factor, and never has been one. Its all hype.
The truth is that even though platinum/iridium plugs can "outlast traditional copper plugs by up to 3 times", plugs deteriorate beyond their point of efficiency relatively quickly, sure you can still get a spark out of a set of a 105k mile plug (stock plugs have that kind of a service life, says so in the helms), but would you really want to run your plugs for that long? After about a year or two, any plug falls beyond their "good performance" and into the "poor performance" range. They will still be able to do their jobs, but nowhere near how a fresh set of plugs will perform.
So in short, this rant is saying... go with copper, why pay more for the hype, copper is the best conductor, the cheapest, and plugs are easy to replace anyways. And even if you went with platinum or iridium, they will perform terribly after 2 or 3 years anyways. Plus you get a set of new copper plugs every year for the same price as a set of iridiums.