I prefer the iPad Air, as it's lite enough for me.
I carry it in my briefcase or in my hand. It doesn't matter.
The mini is too small for practical use, specially when reading PDFs and editing photos, remote controlling a Server or writing a formatted document (with graphs, photos and tables) in Pages. Even with Spreadsheets.
I also use Notability, but I type with the touch keyboard while recording the lectures. Using the iPad Air in landscape orientation allows me to type quickly with both hands, without having to place them in an awkward position and risking wrist injury or carpal tunnel syndrome.
The mini's keyboard is kind of uncomfortable for two hand typing for long periods of time.
That's my input, results may vary from person to person.