Nix has more packages , by far. Nix also automatically handles the dependent libraries for each package, which is something you can’t do with brew on immutable systems. This means that Nix can install software like espanso, which wouldn’t work on uBlue derivatives otherwise.
I really wish the uBlue maintainers would have opted for Nix over brew for that reason. It’s not much more difficult to do nix profile install nixpkgs#package-name over brew install package-name. They could have even aliased it to make it easier.