To be honest although I like diecast, you can't really tell you've got a partly metal figure unless you handle it so I'd say it's probably not worth spending extra on if there's a plastic option..., I just like the idea of metal being used on Iron Man I guess.
Have you thought about looking at the new version of the mark III suit? That is a real beauty & well worth considering if your looking for no version in particular..., it's diecast too. I'd say the mark 43 is worthy of consideration though, the base is very cool & it made the decision to preorder it all the more easier for me, I collect Iron Man figures though so they all appeal really.