Supervisor is solid and mature, that's for sure. If not for the sockets feature, I would say that you would want to move to Circus if:
- you need python 3 - you have a complex start/stop sequence where you need to add some custom code - you manage many servers and want to quickly jump from one to another in the same web dashboard / CLI.
Otherwise, they both provide roughly the same things I'd say
This was probably a wrong or overly simplified conclusion, but I know supervisor and it hasn't let me down yet - so for now I see no compelling reason to switch. Maybe it's one of those "you just have to try it" moments? :)