If we are comparing diecast Iron Men with each other, I'll say each has their own style and none are better than another. (Except maybe MK3 where it's undersized)
MK2- Silver with rivets
MK4- Most iconic movie Iron Man look (can also have the gantry version)
MK5- That awesome scene with the folding armor from suitcase
MK6- Battle damage look of MK4
MK7- Packed full of weapons now also with a pod
MK42/ 43- Fits inside a Hulkbuster
etc.
I'd say get this if you like the look of it and get other MK for their own thing.
Hard to compare.
For the MK 85, it's close to the MK50.
I chose to get the MK 85 as HT Iron Men are $$$