It is important that the method you choose is one that you can sustain. Think about your personal preferences and my biggest recommendation is to download free ones to play around. Notability does come with a small fee, but for myself I find that it is worth it. Further, any WRDSB iPads have this app preloaded (which is great for those within our board).
I think it is important to note that Notability is not the only way that we document. We often find ourselves with pencil and paper (which I then sometimes later input into Notability), I use videos and photos, and monthly we use a class profile to plot out what we know collaboratively about children.
Things that I love about Notability:
-very user friendly and easy to operate
-everything is very quick and instant (e.g., you can add a photo while typing in the file without exiting the program)
-you can organize files and have a file folder with digital files for each student or for each project
-you can quickly send notes to your desktop computer
-you can create one file for multiple students
-it is NOT web based - so you do not need to rely on wireless access/speed
Challenges of Notability:
-personally don't like to format and organize my documentation pieces using Notability (however the new update offers some features that may make it easier to do this!)
-it doesn't automatically sync with your computer, you have to send files that you may want to work on further (again - the new update has fixed this problem - see below!)