I think he's trying to decide between the Iron Man figures that are currently available. So keep in mind that the Mk VI pictured here is the reissue of the older plastic Mk VI and not the sold out Diecast one, which are vastly different figures.
If you could absolutely only have one, I would pick the Mk VII Diecast. But as others have said, we don't know how good the final product will be. Though I can't imagine it'll be a problem as their Diecast Iron Man figures tend to be solid releases for the most part.
I love the Mk 42 Diecast but it's a bit undersized. The plastic VI is definitely not worth it because it's a very old figure that's dated in design and scale.
I'm going to personally recommend getting the Mk IV Diecast now while you still can. Don't hesitate on it. Just do it. Because the prices will skyrocket once it's sold out. The figure itself is pretty close to perfect, and the design is a timeless representation of the MCU as a whole despite not being quite as iconic as the VII.
If you decide you would rather have the VII after in-hand impressions are out, you can sell the IV and get your money back or more.