To me, the plastic ones looks cheaper (in hand) and psychologically there's something about metal in armour... it just sits well, in my head. I don't get the plastic armours. I love the Starboost design, but then again, maybe it's my mind trying to save my wallet I don't get all the DC, only the ones that I like.
Maybe you're right about durability, but I never worry about 10 years in the future, I worry about now. When I enter my office and see these toys, it brings a smile to my face.
Nobody can really tell on the shelf i have had friends think DC ones are plastic and plastic ones are DC (it is still the same paint over the plastic/diecast). When all of my preorders come in i will have well over 30 IM figures and i have every DC iron man Hot Toys has made and i still prefer the plastic ones. Easier to pose, handle and not as quick to try and fall over. I just get the figures i like and I do not really care about the material it is made out of but that is just me.