I have never used nix or nixos. I liked their shared idea (functional, atomic, reproducible systems), and so when I looked at their differences they seemed to all be pros for guix:

1. Clearer, more robust, more centralized documentation
2. GNU Project
3. Guile Scheme (Lisp) as opposed to Nix DSL
4. Unparalleled emacs integration

The only bittersweet aspect of guix compared to nix was the foss only stuff, as I do need some proprietary drivers, but nonguix is so easy it hasn't been a practical issue. And of course I am big advocate of free software so I like that guix is pushing that forward.

There's also a theoretical issue that guix has less packages, but the standard channel + nonguix has had everyhing I use.