The iPad Air launches with 64gb. If you're okay with that, get the Air. It's the better deal.
But if you think you need more space, and you bump up the storage, you suddenly find yourself not that far below Pro prices. And then you get a ProMotion display (which is a luxury, but to me it's the nicest luxury in an Apple product), the thinner bezels, FaceID, and slightly better speakers.
Also - I'm not sure how much memory is in the Air, but the 2020 Pro has 6GB. This isn't important now, but if rumors are to be believed, it'll be important next year when FCP and Logic might launch for the iPad.
So depending on your use case, I hope this info helps.