I use both! systemd starts and ensures circusd is running at all times. Then I can have circus running as an unprivileged user and have jenkins not need to be root to manage deployments. Look up a bit for my other comment describing (from a high level) how this is setup.

Mostly portability: you can provide a single configuration to manage your stack no matter what the system is

It's also easier to run in the user space if you are not root