I've owned ipad 2, ipad mini, ipad 4, and ipad air, and have taken a lot of handwritten notes in Notability with all of them, daily. While I loved the size of the ipad mini, I had a few complaints with taking notes.
I am not sure if it was just the specific ipad mini that I had, maybe there was a defect, but the extreme edges of the screen did not seem very responsive when using the stylus. I thought that maybe this had something to do with the touch rejection on the edges of the screen near the bezels. Regardless, this made the edges of the screen difficult to write notes, thus making the already small writing surface even smaller, and not ideal for taking notes across the entire width of the page. I dealt with it, and used the mini until I lost it on vacation (big bummer) and replaced it with an ipad 4 until the ipad air was released.
Another issue I had with the ipad mini was screen sensitivity in general. It did not seem that the screen was as responsive when using a stylus, at least not as responsive as the ipad 2 and ipad 4 are. When I purchased the ipad air, it also seems that this screen is a bit less responsive when using a stylus, but better than the mini.
Tip: most recently on my ipad air, I started out using a bamboo stylus because I favor the finer tip on these styluses, but it simply did not work very well, I almost had to PRESS down on the screen with the stylus to take notes. I switched to a cheap Belkin stylus and it works MUCH better, perfectly actually.
One other thing that bothered me on the mini, at least while taking notes, was the screen size. I know that the smaller form factor is what makes the mini an attractive option, but the writing area was just a bit too small for my liking when compared to the full sized ipad models (especially considering the above caveats). Annotating PDF's is much better on the full sized ipads too, much less zooming and scrolling involved. The ipad air is the perfect ipad for me, it is very thin and light, with a very compact form factor, much like the ipad mini, but the added screen real estate makes it the better note-taking ipad in my opinion.
