The only guidelines I would give are rather obvious, which are:
- if you care strongly about screen size, get 12” iPad Pro
- if you care somewhat about screen size and portability, get 11” iPad Pro
- if you care somewhat about screen size and portability and price, get iPad Air
- if you care strongly about portability, get iPad mini
- if you care strongly about price, get regular iPad
- if you care strongly about screen size and portability, get two different iPads because those are opposite things
- if you care strongly about screen size, portability, and price, you are screwed unfortunately
But everyone knows this. And beyond these factors, there are a million other factors. Ultimately, how much an individual cares about all these factors determines which iPad (or iPads) is best for them. Unfortunately, no one else can answer this.