GoodNotes is the cheapest of the three applications at $7.99 for a one-time order for all three iterations of the app. But it is the cheapest GoodNotes that generate the most extensive files of these three applications. You would need an iPad with much greater storage if you intend to use it as your critical note-taking app. iCloud backup helps, but it does not fully replace a vast memory on your iPad.
Also, GoodNotes is the best multi-tasking app. If you also want a PDF reader, this is very important since GoodNotes is indeed the best PDF reading app of the three. It's a simple PDF reader, of course, but that's probably all you need to read a few items and highlight them. Many instances and several tabs are enabled by GoodNotes, meaning you can view a lot more documents in the app than you could ever in the other two applications. Another helpful feature for reading PDFs, especially for studying.