When comparing Parabola GNU/Linux-libre vs Plan 9, the Slant community recommends Parabola GNU/Linux-libre for most people. In the question“What are the best operating systems for advanced users?” Parabola GNU/Linux-libre is ranked 7th while Plan 9 is ranked 22nd.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Protects your freedom, certified free by the FSF
Pro Option to use OpenRC or Systemd
Parabola has an OpenRC edition and a Systemd edition, so users can choose which init system they want.
Pro All the benefits of Arch Linux, only 100% certified free software
Pro Very Secure
Pro Has its own repos
Parabola has its own repos (which are almost as large as Arch Linux) and the IceCat browser is included.
Pro Can be migrated from Arch Linux without a CD/USB
Pro Based on Arch Linux
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 Won't work out-of-the-box on the most PC's
Almost all PCs need non-free drivers but Parabola has none to offer.
Con Not compatible with AUR
Unlike other Arch-based distros, this free distro doesn't support AUR.
Con Same cons that apply to Arch Linux
Con Small repositories
Since Parabola is a completely free distro, there is no non-free software available and the community repositories are small compared to the AUR.
Con Incompetent developers
They only care about removing "non-free" software, rather than making a properly working OS.
Con Serves no purpose
It's just Arch without non-free software and is basically a niche OS with few users.
Con Not really free
People have to pay hundreds to maintain this distro.
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.