When I was deciding between Nix and Guix, perhaps 5 years ago, I definitely got the impression that Nix had more contributors and the more comprehensive package set, which is one reason why I ultimately decided on NixOS. I couldn't say which was more up-to-date, but, well, anything not up-to-date is an excuse for me to contribute :)

That being said, at the time I had wanted to choose Guix over Nix. The channels seemed a bit better thought-out and grafts sounded like a good idea.

Not having a lisp background though, it was just easier for me to get into Nix.