When comparing Parabola GNU/Linux-libre vs Debian Unstable, the Slant community recommends Parabola GNU/Linux-libre for most people. In the question“What are the best rolling release Linux distributions?” Parabola GNU/Linux-libre is ranked 20th while Debian Unstable is ranked 29th.
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 Great for Desktop use
It is great for your daily desktop Linux due it good balance of stability and recent packages - it is also recomended by the debian project as the distro you should choose for desktop systems.
Pro Up-to-date software
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 Same cons that apply to Arch Linux
Con Not compatible with AUR
Unlike other Arch-based distros, this free distro doesn't support AUR.
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 Development version
Debian Unstable is intended as a development release, and as such is much more likely to break.
Con Not for users new to Linux
You need to know how to maintain Debian Unstable or you can easily bork your system. There isn't very much information available on how to maintain Debian Unstable either. I was fortunate enough to be involved in a now defunct Linux distribution where a forum member put together a how-to on how to upgrade Debian Stable to Debian Unstalble, as well as a separate forum where users could post tips or ask questions about Debian Unstable. One source I would recommend is the Siduction forums.