I do think 64gb is the sweet spot. I ended up with a 128GB since that model was eligible for a promotion and the 64GB was not. This made the 128GB cheaper than the 64GB at the time. In any case, I don't regret getting 128GB, especially as this will be my tablet for the next couple of years, but it is more than I need right now.
With that said, because of all this storage, I am a bit gluttonous with the stuff I load on there. I've got about half a dozen 1080p movies, a few seasons of TV shows, more than a dozen 1GB+ apps, and I still have about 20GB to spare. I couldn't fit this on a 64GB so I am 'using it', but I certainly could have gotten by without it and been more reasonable with my usage.
Anyway, what you use and want from storage is up to you. 64GB is plenty for a tablet, especially since a lot of content (Netflix, Amazon Prime Video, Spotify) and files (iCloud Drive, Google Drive) are accessible through the cloud so IMO there is less need for local storage. But if you use your tablet in a way that you think you need 128GB then go for it. Someone I know would regularly say 'better to have it and not need it, then to need it and not have it'.