Guix has lots of packages in the default channel, and you can add any odd git repo as another channel providing extra software.

There are quite a few popular channels, such as non-guix (with things like vanilla Linux), guix-science, guix-past, etc.

As a maintainer of R packages in Guix I'd also like to point out that many R packages in Nix actually need more work to build them, so the number of packages in Nix is rather inflated. Guix also goes to great lengths to actually build things completely from source, such as Java packages, or to minify JavaScript from source files.