Earlier quoted context omitted.
Guix is becoming increasingly ready for prime time. There's one big HPC install in Germany already running it. I think it's past the toy stage. I guess Guix + Hurd is a great setup, with innovative features in the kernel and userland. Both very pure in their respective ways, with Guix having functional package management, and Hurd keeping the kernel minimal so that crashes or security breaches are not catastrophic.
What's the difference between Nix and Guix anyway, and which is 'better'?
It's a toss up as to whatever you like. I think Nix has a bigger community, more packages, etc. But Guix, strictly speaking I think, is a 'superset' because changes from Nix often flow into Guix (and Guix can use Nix packages directly), as Guix is built directly on the same source-code as Nix - but not the other way around. There are some other differences, like the fact Guix uses GNU dmd while NixOS uses systemd, etc etc.
It's all just personal preference I think. I use NixOS because it's reliable and has a decently sized community, and a lot of packages. It also has really, really good Haskell support, and being a Haskell developer, that's a pretty big plus to me. Having non-free packages isn't so much of a stickler for me. I think the Guix people are doing good work, though, and a distribution for free software and the GNU project is very important - so I wish them the best even if I'm farther away.
You'd have to try both of them and make a decision for yourself IMO. But I warn you: the rabbit hole is deep, and will require learning. And when you come out - you may be immensely displeased with the current state of affairs. :)
(Full disclosure: IAMA NixOS developer.)