Beancount is both, but you can do either one or the other or both. In particular, you DONT need to learn double entry to do plain text accounting. Of course, you SHOULD learn double entry accounting because it's a great tool for organizing knowledge.
Whether plain text is beneficial for accounting is less clear cut in my mind. I think plain text is backlash against the over-reliance on "services", "clouds", and "lock in", but I think it's misguided. If you're concerned with someone locking you in, it's perfectly concievable to do double entry accounting which lives only on your computer and manipulated with software that is under your control (read: free software).
So in summary, my opinon is:
- accounting: important
- double entry: important
- avoid clouds and ventor lock in: important
- avoid proprietary file formats: important
- plain text: unimportant
(I use GnuCash. It's not without its flaws, but it's great if you agree with the above views)