Conceptually, Cython is mainly for accelerating Python code, and can _also_ access C code. Meanwhile CFFI is specifically for calling C code and nothing else. I recommend the video for the differences.

One concrete thing that pops to my mind is that Cython doesn't support Py_LIMITED_API which means that you need to ship a lot more binary wheels. At least the issue is still open (https://github.com/cython/cython/issues/2542) and Cython projects IME need new wheels for each minor Python release. Compare that to cffi projects that (musl & pypy aside) only have to ship wheels for one Python version / architecture: https://pypi.org/project/argon2-cffi-bindings/#files