When comparing NixOS vs Plan 9, the Slant community recommends NixOS for most people. In the question“What are the best operating systems for advanced users?” NixOS is ranked 8th while Plan 9 is ranked 22nd. The most important reason people chose NixOS is:
Atomic non-destructive upgrades / rollback of a system upgrade / declarative reproducible system configuration / unprivileged installation of packages / transparent source or binary deployment.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro State of the art package manager
Atomic non-destructive upgrades / rollback of a system upgrade / declarative reproducible system configuration / unprivileged installation of packages / transparent source or binary deployment.
Pro Minimal
You can start with a minimal environment and add packages and software to suit your needs as you go along.
Pro Reproducible system
NixOS is configured using the Nix package manager, allowing your system to be replicated and kept in sync across multiple machines. Great for keeping a laptop and desktop in sync.
Pro Robust
Packages don't break after a NixOS upgrade as they are prone to with other distros (especially Arch).
Pro Well designed
Plan 9's design is very orthogonal and well done.
Pro Designed to allow for multiple computers to work as if they were one
With Plan 9, computers are turned into terminals where you can access files and processing units which can be located elsewhere. Data storage is handled by another external server. This means that instead of each person having their own individual computer with all the required parts, all of the computing and storage is done in a central area and everything is networked in together.
Cons
Con Documentation is not good
A lot of the documentation of various functions is buried on the source code, their respective manuals, or non-existent. The documentation, the conventions, and the scattered toolchain really made searching for stuff easily missable.
Con A configuration change might end up bricking your system
Con Difficult to use software made for other systems
Plan 9 is very different from most other operating systems, and as such it's extremely difficult to bring in software designed for other operating systems ('porting').
Con No fully featured web browser
As indicated on the official Bell Labs webpage, there is no full featured web browser currently working on Plan 9, except abaco.
Con Not supported by most tools
Not supported by most dev tools beyond text editors.