I honestly believe that Guix will eventually pull out ahead of NixOS, simply because its going to be part of Debian 11. NixOS is a bit easier to set up and use for now, especially for provisioning simple linux servers around commonly used packages.
One interesting architectural difference between Guix and Nix is there is no separation in Guix like what you see with Nix and nixpkgs. Guix and the packages are all just one massive repo of scheme. For Guix this tighter integration can be a development advantage. I cannot imagine how this can benefit targeting multiple platforms, which Nix is able to do with Linux, Mac, FreeBSD (as of 2020), yet somehow, Guix is able to pull off cross-platform support for GNU Hurd.
What separation is there in Nix? The whole system is just a big derivation the same way as a small package is.