We see bias with most discussions.
Only cons with Guix I see is, lack of infrastructure and less volunteers to work on guix eco-system. If its solved, I can imagine guix can improve exponentially.
Nix is pathologically recursive, lazy, and uses fixed points, things that are very apt to changing something that cascades through a bunch of dependents. Nix's runtime is not magic. Guile should be able to expose a language and evaluate it in a similar way.
For my part, I've not opted into Guix because it's a GNU project, and I've decided to avoid anything in the FSF sphere of influence. Their orthodoxy turns off contributors and they have a history of taking insular hard-liner approaches that are utopian. Outside of coreutils that are about to be fully subsumed by rewrite-it-in-Rust (which has a community that is not a fan of the GPL), what has had FSF backing and been successful? Linus starts two of the most influential pieces of software in human civilization and RMS wants to name the awards. The pragmatic culture that shifted away from the FSF has I think largely adopted Nix, and it shows. Nix is open for business, available on lots of platforms, has commercial entities built around its success.
GCC is still indispensable. I doubt it will be rewritten in Rust any time soon.
"Best" doesn't matter, you just need a seed crystal that's good enough.