My iPhone I get a 128GB because I want to put all my music on it. It goes with me everywhere, so you never know when you might want all your music, and I'm not paying for Apple Music as of this time. Plus, I never know how many pictures I might take in places without a great connection or an expensive connection to upload, I can keep it all on the device.
However, on the iPad, I don't need to store any music. It might be fun to edit pictures someday, but I probably won't, and they don't have to be stored on it permanently.
Therefore, I primarily need space for Apps and 32GB should be plenty.
Still, Apple makes it hard to choose the 32GB and with 90 percent in this thread choosing 128GB, it might be better for resell later should I need to get a newer model.