Iron (or ductile Iron) do have a kind of main disadvantage for some - they tend to rust when in contact with high humidity.
I've to admitt, I never cared about that too much, as Iron discs still provide more stopping power at the same force than stainless steel discs. But of course the choice what to choose depends on the purpose. At mixed weather conditions wet/dry stainless steel discs do have a better calculateable stopping character, compared to Iron Discs.

To put it in one: I exclusivly used Iron Discs past 15 years, because their brutal braking Performance. On my Bikes I never needed more than 1-2 fingers on the Master for really hard braking. But of cours - I hate wet conditions on track or on road, so for me choice to go for Iron discs has been easy always.
If you're right like me, and only ride in nice weather Conditions - Iron Discs would be my recommendation.