First things first, why do you want iridium spark plugs anyway? They cost 4-5 times more than platinum!
In a word consistency.
Unlike platinum sparks plugs whose spark gap grows over time, increasing strain on the ignition system, iridium spark plugs are massively more stable
Depending on the engine and level of tune, you may have the check the spark plugs every 3-6 months to check the spark gap with a platinum spark plug.
Iridium spark plugs with their iridium electrode last many times longer than a platinum spark plug, they maintain a much more stable spark gap and they do not require as much attention ie you dont have to check them so often.
In short, iridium spark plugs last longer and their performance is consistent unlike platinum spark plugs
NGK Iridium IX vs Denso Iridium Power
Is you butt dyno going to be able to tell the difference?
In a word, no.
Ultimately the difference comes down to philosophy.
NGK Iridium IX are designed to be a standard replacement that lasts many times longer than platinum while over more consistent performance over the life of the spark plug, and with less maintenance.
Denso have taken the superior hardness of Iridium and chosen to shrink the diameter of the electrode more than NGK. Because there is less material the life of a Denso Iridium Power spark plug is going to be less than an NGK iridium spark plug but the thinking is the smaller you can get the electrode, the easier it is to create a spark which gives the spark more power.
Like I said, you will not notice the difference in power, we are probably dealing with fractions of a percent but if you are looking to squeeze every last ounce of power out of your engine and you dont mind paying a bit extra and having a shorter service life than NGK Iridium IX, then Denso Iridium Power spark plugs are the choice.
If you dont care about fractions of a percent in horsepower and you want a plug that will massively outlast and outperform platinum spark plugs, you need NGK Iridium IX.