Which do you prefer, and why?
My writing experience: on my old Ipad 3, my favourite writing app was Noteshelf using Adonit Stylus. I really liked the infrastructre, and was very intuitive when creating new books, changing notebooks, moving pages. However, with the Apple Pencil, Noteshelf has become too 'creative" and changed my writing to calligraphy. This does look nice but I found it hard to decipher - I have terrible writing but can read my own writing, whereas the calligraphed version did not look familiar and although looked good was difficult to read.
Notability: The writing experience is good,but I found the 'filing' system to be cumbersome. The app uses Subject and Divider format, which makes sense when setting up but very difficult when looking for past notes. The thumbnails are useless and each divider has its own page and tiitle . Its library of templates is limited - some available for download but not available when creating a new page. I even invested in the Mac app but the overall experience did not meet my expectations. Gave it 2 weeks and eventually gave up.
Goodnotes 4 - This comes closest to Noteshelf. The notes look personalised, and the writing experience is good. Navigation is easy, although sync with iCloud is slow - for a while I thought sync to iCloud was not working!
I also use PDF expert - it works great with Apple Pencil and use it on IPad and Mac. Really beats preview when annotating notes or publicity material
Each to his own, and you might have to try a few before identifying what suits you and your needs