Which to use - systemd OR supervisor?

For your use case, systemd is good enough.

systemd pros:

Comes inbuilt with OS. No need of another dependency.

Simple to use. No learning curve as it can be managed like a system service.

supervisord pros:

Any user can manage processes. No need to be superuser.

Has nice web interface to manage process.

Works on any distro.

More flexibility in managing process(groups, priority order).

You can read docs to know more advantages of supervisord.