There is very little actual diecast content in the Hot Toys diecast figures.
But that said...
The Diecast Iron Man figures are simply better figures than their plastic counterparts (meaning DC Mark 6 vs Plastic Mark 6, or Diecast Mark 5 vs Plastic Mark 5 ,etc.).
I'm not sure if that's due to simply the release date (newer figures, newer construction, engineering, etc.), but the Diecast versions are superior in paint, engineering/articulation, detail and proportions versus their plastic counterparts.
So in my opinion, a collector shouldn't be buying them because they are diecast, or because they are heavier. They just look better and pose better.
For me personally, I never owned the plastic ones. (I've done the comparisons in-hand, when looking at the collections of some friends who have lots of Iron Men). So since I never owned the plastic ones, for me, I didn't mind paying for the diecast ones I own because I thought they looked better....