Logically …
Future-proof is an absolute term, but the future is not known. Relatively, more should be more future-proof in this case. Just think the other way around: Is there a scenario in which less RAM would be more future-proof? Fairphone have this option to have a reduced price point on offer, I guess.
Psychologically …
You wonder about this now. The question is there and won’t go away from you unless you go for the 8 GB option, even if there might be reasons in favour of 6 GB. Again, the future is not known.
Else … since you know yourself best … How much or deeply would you regret having chosen 6 GB when it turns out in the future 8 GB would have been better?