When you commit to Postgres and the server acknowledges it, you know for sure that it's been written to disk and that it will survive anything but a hardware disk loss (or, obviously, system/FS bug). When clustering is enabled with synchronous writes, you can also be confident that the data has been recorded to another node as well.
With redis clustering, there's no guarantee the data has been replicated. I'm not even sure there's any guarantee the data you just asked to be recorded be stored even once if a power outage happens immediately after the request.
