It depends on your priorities. If you care about package availability and the broader ecosystem, Nix would be a better choice[1]. Nix has a larger community size, lower barrier for participation, and is more forgiving regarding non-free software. Its package count likely exceeds the AUR too, if you exclude duplicates.
Guix, on the other hand, is better suited for the more adventurous folks. It's built on Scheme, which many find appealing.
Both Nix and Guix can be installed on other Linux distros. Both are now available in the official Debian repository. Additionally, Nix supports macOS quite well. It even supports FreeBSD and Cygwin too, although it's probably not as mature.