I think the AUR is at a slight disadvantage here in that it isn’t meant to be a complete repository containing every package, but to complement Arch Linux’s main repository with additional packages which aren’t in the main repository. While there is some overlap where e.g. the AUR has a different version or flavour of a package which exists in the main repository, it would probably make more sense to consider Arch + AUR together, in that that represents the total range of packages available to an Arch Linux user. If we do a quick and dirty calculation ignoring any overlap, that would give 58222 + 9370 = 67592 total packages packaged for Arch Linux, which is quite a bit more than nixpkgs unstables’ 58617 packages.
(That wasn’t meant to be a reply to @Ekleog, but now I can’t seem to change it and if I delete it and try to re post it as reply to the thread it complains that the body is too similar to what was recently posted 
)