Yea, I think most people feel the proportions are better on design of the IV over the III, but I think Hot Toys accurately captures the proportions on the Diecast MKkIII as they actually are in the film. Even so, I prefer the MKIII over the IV as the most "classic" version of the suit. Proportion aside, the details on the MarkIII seem to evoke more of that "classic" comic look over the IV, imo. it's the small things that make the difference, like the two ports or screws (or whatever they are) above the arc reactor near the collarbone which mirrors a similar feature in the comics. The Mark IV armor doesn't have this and feels like its an evolution from the "classic" suit, which of course it is; even if just a small one.
So I prefer the Mark III for the most "classic" representation of Iron Man and the the Mark VI for a more noteworthy evolution of the armor over IV.