The biggest difference between Nix and Guix is that Guix doesn’t support non-foss software, meaning you can’t use it as a package manager on other operating systems. I originally wanted to use Guix but use a Mac for work, so that became a deal breaker.
Nix is pretty awesome as a package manager, I’ve been happy with it after the truly unnecessary learning curve brutality. I do not imagine I would ever use the full OS though.