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VI would translate to current gen far better, in my opinion. In addition to being the best game ever made, it has a more adult story.
How exactly do you decide this?
Personal opinion: VII hits closer to home, and just generally feels more adult to me.
It deals with a capitalistic dystopia (instead of an intangible fantasy "empire"), rampant genetic modification, the outsourcing of armed forces to financial interests, and the tragedy that comes after you've traded the health and resources of your community away for a quick buck (ties to "Big Oil" and the fracking industry in rural US).
The game picks up in its own World of Ruin. Of the characters who aren't totally washed up failures already, one dies, one has horrible self-esteem issues, and the other is the Princess of a town/country that has been reduced to a tourist destination. Cloud, the badass on the cover with the big ass sword, went away with big dreams of making something of himself and came home the biggest failure of the entire cast.
VI picks up when the Big Bad wins. VII starts off in a world where the Big Bad won a long time ago, and there's nothing you can do about it. It's heavy, and in a way that connects way more to what I consider mature themes and story beats. Not edgy "mature", though there is some of that, but human mature.