S8 never impressed me, neither the Plus version, the S7 edge was great phone until the Nougat update, but even then it wasn't that great...
I would take G6 over S8 any day, it might not look "wow" but it probably works "wow" as oppose to S8, which will slow down very soon for some users, however as i have the Mate 9, i wouldn't buy any phone now unless it's maybe the Mate 10.
It's kinda offtopic, but as i had those three phone, i would say that the Mate 9 is the best phone atm, and it's sooo underrated, but buying G6 with all it's features is a great choice.