We have all of those combos--and, if I were a student again, I'd agree with the suggestion of a used MacBook Air, especially if this is your only device.
They are light-weight (not much more than an iPad Mini-keyboard combo), powerful, and full-fledged computers (meaning that you can run all sorts of programs, including technical ones--e.g., statistical packages).
The iPad Mini--bluetooth keyboard combo works OK, but it means juggling two different items, having to turn on an extra device, and having to prop the Mini at the right angle--which means Disaster City awaits your on those small seat-desks in classrooms, and you're shut out from running major desktop programs. You also could need a foldable carry stand for the Mini-- some of the cases can work, but they can also add bulk. I've also seen ones that are so tippy they can make the Mini fall over.
OTOH, typing on a MacBook Air is like typing on a brick. Almost any bluetooth keyboard--especially full-sized ones--will be more comfortable. The HUGE bezel around the screen on the MBAs is off-putting, too.
There is really only one way of determining which is best for you-- try them in a store, but also, if you can, try them in a classroom and see how it would work. You'll know pretty quickly which would be best.
Bottom line, however, relative to your original question-- I agree 100% with others--
an iPad Mini will be much better than an iPod Touch-- much, much better for taking notes and for just about everything-- save sticking it in your shirt pocket to listen to podcasts while walking! The Mini won't fit!