if you are just doing basic things like reading stuff online/books, drawings, media watching, AAC, etc then an ipad standard is fine. if you are doing something like drawing for money and it is a job a pro can make more sense as bigger display and more layers in drawing. if you have poor vision a larger one could also be better but I think the Air 13 would be better money than larger pro for that requirement.
I am looking at getting ipad soon for AAC. I had an older one but was eh about it not being good for what otherstuff i wanted but i think an new ipad would be good enough.