You can't listen to other people here and base your decision on that. What do you use your ipad for? Are you in range of WIFI most of the time? If so, the 64gb should be fine. Do you play a lot of games, and require multiple games/movies/ tv shows, etc on your ipad? If so then the 256gb would be a better choice. Do you store your entire life's work on an ipad, and actively seek out the newest game to download, and never play? Then the 512gb is for you.
I have always went with the "second" tier of storage. This year, I moved from a 9.7 ipad pro 128gb to a 12.9in ipad pro (2nd gen) 64gb, and I have plenty of space. I primarily live in the cloud, and Google and Onedrive are my go to file sources. So my photos, emails, calendar, etc are with google. While all my traditional files are in Onedrive. I am able to access them on demand, when I need them.
I have a few folders of pdfs synced, locally, to my ipad pro. I also have a series of productivity apps, entertainment apps, social apps, nest, etc on my ipad. I use about 11 gbs. I used to use 48...but then I actually took time to see what I used/wasted space. Thus, I opted for the 64gb storage.
The local file system isn't going to magically increase the amount of files you keep on your iPad. You could already do this, through less conveniently, in iOS 10. The new file system is just going to make it easier. You will still be able to live in the cloud, or store everything locally. It is all up to how you use it.
I consider myself a power user, and am fine with the 64gb. But you may not feel the same way.