When comparing Parabola GNU/Linux-libre vs Haiku, 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 Haiku is ranked 16th.
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 Very fast
Pro Beta has been released
After about 6 years since the alpha version, beta has been released on Fri, 2018-09-28. Check here for release notes.
Pro Only need 512mb ram
Pro Stability
Pro Runs perfectly on old Hardware
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 No one uses it
It's a very niche OS that no one uses.
Con Unfinished
It's still in beta and quite unstable. Making it unsuitable for developing applications of any kind.
Con Language support is terrible
Con Small community
It is important when developing to be familiar with tools that other developers use. You can make any utility in any language you feel like, but if it's in an esoteric language that no one can read targeting a small platform that no one uses, then it was just something you did as a hobbyist, not as a developer.
This is not to say that Haiku isn't a great operating system to hack around on. Just don't delude yourself into thinking you're doing it to get familiar with tools that you need to know to be a better developer.