Keep in mind how you plan to use the iPad. My old iPad Air had around 20GB in use all the time. That included around a thousand of my favorite songs, 4,000 photos (synced with iCloud devices), 200 books, 150 apps. When I traveled I added a few movies, which pretty much filled up my 32GB of storage.
This time I opted for the 128GB model. 64GB would probably have been plenty for me, but that's not an option now. Also keep in mind that the new iPad Pro 9.7" has a greatly upgraded camera that will now shoot regular photos, Live Photos, and 4K videos. The 12MP camera's files are now 50% larger, and each Live Photo is a mini 3 second video. 4K videos are huge. If you ever plan to make use of the great new camera, you'll need the space to store those larger files.
One final caveat, if you ever plan to resell the iPad, the extra storage capacity will likely affect its value in a major way.