Guix is cool but has two big drawbacks:
The major one: package ecosystem. Nix has the largest and most up to date package repo of any distro [1]).
Nix is already somewhat niche, but the Guix community is tiny.
They also have a hard stance against proprietary software, so many things that are packaged for Nix aren't for Guix.
Second: it's pretty slow. A lot of the Nix functionality is implemented in C++ . Guix is all written in Scheme, and uses a rather slow implementation.
The main advantage is more coherent tooling and documentation. (Guix is much younger and doesn't have decades of legacy cruft)