iPad Question

As already mentioned by ckuan, you can directly compare major features of the various models of iPad here:

iPad - Compare Models - Apple

In summary:

• The iPad Pro models represent the pinnacle of performance and features - with a price to match. The iPad Pro 12.9” is the iPad with the largest screen. Only the iPad Pro models offer internal storage of up to 2TB and 16GB RAM.

• The iPad Air is a high performance mid-point, offering much of the “performance” of the iPad Pro - but lacks the advanced hardware features of the Pro models (such as orientation adaptive multi-speaker audio, advanced camera array, LiDAR sensor etc.).

• The iPad9 is the current entry-level iPad.

• The iPad mini6 is a small form-factor device, with the smallest screen in the iPad range.

Whatever model of iPad you choose, you would be well advised to purchase as much internal storage as you can afford - as internal storage cannot be changed, upgraded or extended. The available internal storage is fixed for the entire life of the device. If you run out of internal storage for your documents, photos and other data, you will forever struggle and become a source of frustration. It is far better to have more storage than you need, than to need more storage than you have.

A good rule of thumb is to quantify how much data storage you will need - add a healthy margin - then double it. If you are unable to quantify your data storage needs, it becomes more difficult; whilst I cannot make definitive recommendations, personally I would council against purchase of any iPad with any less than 256GB storage - 512GB or greater offering a greater level of safety.  

Within the Apple Support Communities we see regular tales of woe and disappointment when storage requirements are exceeded, necessitating replacement of the device; by contrast, we never see complaint of having too much available storage. It is relatively easy to fill limited internal storage space; perhaps consider higher tiers of storage as providing room for growth as your needs develop or mature - potentially extending the useful usable life of the device. 

Consider also the functional differences between WiFi Only and WiFi+Cellular models of iPad. Only WiFi+Cellular models include GPS capabilities - which may impact any Apps or features that require accurate and consistent location data, such as Mapping. Purchase of a WiFi+Cellular model does not commit you to using Cellular connections, but provides this flexibility should it be needed.  WiFi Only models of iPad have no Cellular or GPS functions at all.

I hope this information and insight proves to be helpful to you.

You can look and compare them at Apple website.

Read here => iPad - Compare Models - Apple