In the film you see the mkVI utilise some different gadgets, some which its pretty safe to assume were present on the mkIV but there just wasnt the opportunity for them to be used. Others were designed for the mkVI which I think he mentions specifically such as:
Those laser things on his wrists that just wipes out the hammer drones. But those would be a bit pointless to include I think since it would just be a little red light on his wrist or something
If I was more of a completist I would get both but one version is enough and the mkIV design just hits the spot more for me than the mkVI does. Not saying one is better than the other, I just prefer it. I also prefer that with the mkIV you get a headsculpt of tony not in the helmet so theres the possibility for a stark in business suit kit bash with a better sculpt than that which came with the mkIII.
Also since its pretty safe to assume that the headsculpt which would come with the mkVI will be a helmet one with flip up visor, like Ingo (The Entity) said, will most likely be based off that final scene since he doesnt remove the helmet completely whilst in the mkVI. Then short of Hot Toys doing a different paint job like with the mkIII and mkIIIBD, you could just purchase that spare head and you will have that option also for the mkIV for those type of scenes in the film where he just flipped the visor or whatever. Unless ofcourse there are some differences in the helmet that I havent noticed.
Appologies for rambling on or if I dont make sense