Seriously, I think there's some serious overlap between these two. To wit, a lot of people have a 12.9 Pro for heavy work and multitasking, and then they want another iPad for portability. Before the Mini 6, their main options were either the 11" Pro or the Mini 5 or regular iPad 8. Here are some of the factors that influence that decision:
- Desire to share the same Apple Pencil 2 between bigger and smaller iPads.
- Desire for portability that the 12.9 doesn't have.
- Wanting a modern all-screen design.
- Affordability.
Before the Mini 6, any choice involved major tradeoffs.
If you went for the 11" Pro as a second iPad, you got the all-screen design and got to swap your Pencil 2 between the 12.9 and the 11, but you lost out on portability and affordability.
If you went for the Mini 5, you got a small form factor, but you gave up the all-screen design and had to buy a separate Apple Pencil 1.
With the Mini 6 as a second iPad (companion to the 12.9), you get closer to having the best of both worlds, albeit with a few compromises (e.g. lacking ProMotion and FaceID).