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.
Im not sure. Ive switched from nixos to guix, and i think nix's main advantage is just "a **tonne of code" for a wide variety of services, but guix allows new services and things much easier imo
I heard somewhere that they are going to include it in the distro, so no guix deploy afaik, just the package manager itself as a package you can install. This should bring a lot more attention to the project at least.
Guix is now in debian and apt-installable in ubuntu 21.04. Installation was the biggest hurdle, and there are still some quirks even with the debian installation--- you've got to set up your shell environment correctly to use it--- but in general it's super easy.