Nix is what Guix is based on. You can think of Guix as the 'GNU Version' of Nix/NixOS, meaning it provides free software packages, and IIRC it uses Guile Scheme, rather than Nix, to describe all the packages and OS configuration.
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. :)
I'm giving both a try. Nix is more polished, has many more packages, and is better documented.
I prefer Guix choice of using a real programming language (Scheme!) instead of a DSL, but I really like Nix anyway.
Something that annoys me sometimes is that Nix has a few really bloated packages, compiled with all dependencies on. For example, if I try installing mutt I eventually get python as a dependency. This is a bit ugly. I'm aware it's easy to change this, but I'd still love to get thinner binaries from hydra. Otherwise, a really neat piece of software.
Guix tends to be a bit more like Slackware or Arch. Very vanilla things. I would love if Nix went a bit in that direction too with regards to packaging policies. It's more secure and nicer to humble devices, like cheap Chromebooks.
