I have not tried runit. s6 is actively maintained; I don't know about runit (on the other hand, runit is in Debian while you must compile s6 yourself). The author of s6 has a (very biased of course) comparison or various supervisors:
The big feature s6 has that runit lacks seems to be the ability to wait without polling on some event (process started). I haven't used this much myself, but even given that some process has started, it doesn't necessarily mean that it's initialized and ready to do what you expect.
