$ nix-env -iA nixpkgs.ungoogled-chromium
$ which chromium
/home/sergeykish/.nix-profile/bin/chromium
guix install ungoogled-chromium
For example, the Nix language doesn't have the ability to define new data types, and it doesn't have a "package" or an "operating system" data type. That means that all the entries in Nixpkgs look alike, which in turn prevents building a "package-aware" or "OS-aware" user interface.
The Guix CLI "knows" about packages and versions, and package transformation options are an example of how the CLI can take advantage of that abstraction.
Likewise, the "guix system" command knows about services, which gives it convenient introspection capabilities such as "guix system extension-graph" and "guix system shepherd-graph".
It really isn't about parentheses vs. braces. :-)
Guix on the other hand provides those, but is far from what nixpkgs offers in terms of number and actuality of pkgs.
Nixpkgs has the biggest package repository of all the distributions, and a much larger community.
If you compare Guix against Arch for example, Guix comes out more favorably (15k packages) vs Arch's 12k.
Guix packages tend to be very well integrated too, usually running upstream test suites, leading to fewer run-time problems. I think only Debian and Fedora matches Guix on this practice.