Is Iridium Spark plug more efficient with respect to double-platinum? which one last longer?

Iridium is harder than platinum, and in most cases, iridium spark plugs last about 25 percent longer than comparable platinum spark plugs. If your owner’s manual specifies iridium spark plugs, don’t downgrade to platinum spark plugs or even copper spark plugs. They won’t perform as well as the iridium spark plugs in this case.