Ultimately, it depends on what you use it for.
My iOS devices are primarily used for e-mail, light web surfing, some information-access apps (like IMDB) and puzzle-type games (like Homescapes). These apps don’t use a whole lot of storage. While my old iPhone (with 16 GB of storage) was very cramped, the base 128 GB of the current iPad Pro would be far more than I need.
It’s only media files that consume a lot of space for me. Which is why my iPod Touch has 128 GB - because it’s holding my entire music collection, which is about 65GB.
Of course, for what I use iOS devices for, I would never get an iPad Pro to begin with. I’d stick with the basic iPad or iPad Air. I would probably avoid the minimal 32 GB storage, just to be safe going forward into the future, but I’d take the next size up from that (or maybe one notch further if I plan on using it to hold media files).