For me personally, as a person who's tried Nix, it's helpful that the Nix language is simple, self contained and easy to learn & grok quickly. I never learnt Scheme, and its use in Guix, though extremely interesting and appealing in theory, also feels notably overwhelming to me. That I'd have to learn a whole (presumably huge) R7RS or something, just to be able to use Guix. And then still have to learn the Guix "API" or DSL over that to be able to actually use it. While the Nix language is small and fully described in the Nix manual, in surprisingly few words.
Disclaimer: as a Guix co-maintainer I'm totally biased.
We don't use R7RS in Guix. You need to know about the Scheme syntax, obviously (including keyword arguments), and a couple of common procedures like `string-append`, but aside from that you don't really need to know much about Scheme at all.
What comes in handy is the Guix DSL, which provides a convenient way to specify packages and download origins. Guix also has a bunch of utility procedures that are useful extensions to their Scheme counterparts, such as `mkdir-p` (which does what you think it does) or the `substitute*` form to substitute expressions in a file or list of files.
One important difference between Nix and Guix is that Guix does not glue shell snippets together, but eventually compiles to Guile builder scripts, so it's Scheme all the way down.
Does Guix let you write interactive programs in Scheme as part of the system configuration? For example, defining a systemd service that uses Guile libraries and so on, as a subexpression of your OS configuration file? While also referring to shared variables like the system's hostname, etc?
I skimmed the paper on "Code Staging in Guix" and I think this should be very doable, but I haven't yet tried Guix for real.
This ability seems like it would have huge implications for system development... I've dabbled with such experiments using Nix, but the lack of hygienic code staging makes it a bit icky.
I don't know about interactive programs; in Guix we like system configuration to be non-interactive.
We don't use systemd but the Shepherd, which is written in Guile Scheme, so system services are indeed written in Scheme and can use Guile libraries. It is a common pattern to define services as the result of procedures taking variables as arguments.
System services in Guix don't have to be limited to Shepherd services. The system service framework in Guix is very flexible and can be used for almost any system setup task, even for mere "activation services" that create directories or files. Services can extend other services:
