As someone who has tried both, I found the learning curve for Guix much more pleasant than for Nix. With Nix, the community is for sure a lot larger, and more things are available (notably it is a lot faster, too). But I could never get everything clear in my head, how it hangs together, and the community seemed very much more fragmented than with Guix. I think the Guix documentation and manual, as well as the CLI and secondary docs written in people's blogs, were much easier to get a handle on than with Nix. Flakes are a great example, I still have no idea what a flake is, they are still an experimental feature, and I have no idea how I should think about them. I've been using NixOS full time for 3 years by the way.

I also think the shepherd alone is almost a killer app relative to something like systemd. I had to write a service myself for restarting ssh tunnels, and I could even do "advanced" things like templated configuration, almost trivially. It felt like a superpower, relative to systemd services.