I would say play around with each. I prefer OneNote for a few reasons. It has better organization with notebooks, tabs and and timestamped pages. Notability felt like a PDF markup and drawing app... It was difficult to input text well on the iPad when I took genetics but I excelled in my optics class when the professor chalk-talked for two hours. OneNote is pretty slick on OS X and iOS -- feels a bit more professional and organized and saves everything to their cloud automatically, which is awesome. I had issues with Notability and my Google Drive, but that was back a few semesters before they had a desktop app (which I did not download due to poor reviews). For OneNote, you can download all the apps for free. Notability is a few bucks on each platform, but no more than $10 combined -- worth taking a shot at each.
tl;dr: IMO Notability just does not feel like a "polished" enough app for my needs. OneNote offers better cloud saving across OS X/iOS, better note-taking functionality and better organization.