"Note taking" is an app category that I have played with before, but never taken seriously because the dots did not all line up -- there was insufficient feature coverage. Now that I have that, "note taking" (and syncing, and organizing, and search....) has suddenly become a veritable killer app. I use it constantly during the day, obviously more than just note taking: I am building a personal knowledge base.
I might end up using both of the subject "note taking" apps, but I am currently leaning strongly towards using Joplin for almost everything.
What they have in common: multi-device support, flawless syncing between devices via an external server, open source clients, Android apps are both in Fdroid, point-to-point encryption between devices, nice UI's.
One thing that is really different is that Standard Notes is backed by a company and has a paid level of service. However, one can still run one's own Standard Notes server if desired.
Why I am I going with Joplin? Two reasons: because it lets me use my own Webdav server, which is very easy and so far works very well, and it lets me attach files to notes. Although Standard Notes' own custom designed server is open source and can be installed by anyone, the instructions are pretty complicated, while the simple Webdav option is buried in the paid level of service. As is the ability to attach files.
(Note that Webdav is a little bit flaky if you switch back and forth between multiple Webdav servers, but rock solid if you stay on one.)
Why I might continue using Standard Notes at least a little bit?: It has more security features. Namely, client-side on-disk encryption, and optional PIN locking of the UI. The locking is granular, one can lock some content and not lock other parts, and fingerprint unlocking on mobile devices is supported where available.
For anyone who does not care / can not be bothered to run their own server, really **WANTS** that extra bit of security, admires the simpler UI, and / or does not care about attaching files, free Standard Notes is really a viable option. For that matter the paid level of service is not that expensive, and then you get almost complete feature parity with Joplin, and zero headaches.