[taylanub]

Guix is a GNU project, meaning it will adhere to a higher code of software freedom ethics. (For instance Nix ships Adobe Flash Player with Firefox if I remember correctly.)

Guix uses Guile Scheme APIs/EDSLs for package recipes, build systems, etc., giving you the power of a proper programming language (and a lisp at that) as opposed to the special-purpose Nix language and arguably dirty Bash and Perl code.

The Guix System Distribution uses a Guile-based init system, DMD, instead of systemd, which again should give a lot of power and flexibility. (systemd being implemented in C instead.)

That's all off the top of my head. I haven't used Nix and someone who did might be able to point out many more differences that aren't obvious at first.