So far a clear win for #Nix over #Guix for me: it's very easy to run build steps from a shell. I don't have to jump through hoops, like adding a deliberate error after a build phase in Scheme and enabling keep-failed and then entering the build environment and copy-pasting commands from the log to re-run them.
None of that nonsense, you just enter a nix-shell '<nixpkgs>' -A pkgname, set out to some output folder in /tmp, then run your build phases, or a generic build.
Replacing Bash with Guile could be a clear win too, because Bash is an awful language, but Guile's interactive experience is not all that great in my experience, and because of how code staging is done in Guix, you can't just easily run bits of Guile code inside an interactive REPL to build a package.