I don't know a lot about plastic quality and haven't really handled any of the 1/6 toys besides Silver Centurion. That said, this feels very solid and sturdy; I have never worried that something was going to break. The joint tolerances are also very obvious—for example, where the older 1/6-scale Iron Man figures have an elbow with a hidden joint which you might break, this is a visible, clicky double-joint whose limits you can see easily. I know the knee joints (the joints on the back of the knee that you can see and touch) are made of metal; unsure if there is metal anywhere else.
Also: I personally wouldn't set too much store by what is die-cast and what isn't. Metal joints can still be poorly made and hold a pose badly, so die cast content is not synonymous with quality or even toughness at all. For example, my Fewture Getter Poseidon is mostly metal, down to the joints, and yet it still has some floppy, badly-assembled joints. Besides, the paint will degrade all the same, you're likely to suffer more chipping, and some people here have said it is easier to achieve a nicer paintjob on a plastic figure.
Anyway, if you have a chance, get this figure in-hand and move it around a little, and you will be able to make up your own mind. For me, I would prefer a very solid plastic figure of this size to a die-cast release. I get tired lifting the Mark 43 up for long periods while posing, and the added weight of metal would make it even harder.
), 
(worth it)
