I've used s6 a lot, though not as an init replacement in any systems that matter. As a supervisor I think it's great. For basic stuff (root process supervising supervisors supervising daemons) the two are basically identical with some superficial differences. For wider ranging stuff s6 is really good since it has a bunch of ancillary programs that solve a lot of serious issues in full system supervision (readyness notification vs polling in a run script, ucspi socket handing, etc).
I missed actually answering the question about the difference between the two (plus, that comment is in dire need of editing and the edit window appears to have expired).
The superficial differences are things like: service to logger pipe holding happens at different levels of the supervision tree (the root `s6-svscan' holds them in s6, the per-service runsv holds it in runit), `s6-svc -CMD behavior cannot currently be overridden whereas you can with `sv CMD', `s6-svscan' will immediately re-scan its directory with SIGALRM whereas `runsvdir' only polls for changes on a 5 second timer.
For basic supervision tasksboth are great, with runit being the simpler of the two in terms of understanding what it gives out of the box. For larger tasks (full system supervision, inter-service ordering dependencies, etc) s6 has the tools to make that easy whereas with runit you're going to find yourself playing stupid tricks in run scripts to get similar behavior.
