I have the 11” pro, not the newest one but the one before that. Reading programming books, docs etc is the thing i use it for most though I do use it for other things as well. I find it to be really good for this task since it can render the graphics and diagrams nicely, support any format, easily open links, bring up docs/book side by side in split view, weighs less than most of the books I have in dead tree format…
In general unless you have a really good reason, I would be inclined to go for the Air rather than the Pro. For this kind of use case (and realistically most use cases) the pro doesn’t add enough to justify the extra cost. I would however recommend picking up the (2nd gen) pencil, it is great for notes, highlighting stuff or quickly sketching and it feels really nice to use. For me the mini is a bit on the small side for this kind of content, it is purely subjective where the size trade off is right for any person though. I like the size being similar to that of a normal printed page.
If you also want to do some heavier image editing (affinity photo and designer are great), video editing or get into some of the audio tools then you might start to see the benefits of the Pro, it is a pretty awesome bit of kit but it is also fairly hard to use to its fullest other than with these kinds of apps.
Those are my thoughts. If you can go into a store and play around with them for a little bit that might help a lot. You spend a lot of time holding the device so getting a feel for the sizes and weights directly is a good idea.