They are both quite different as its an internal DSL vs an external one. Just give them a try. A major difference at the minute might be package availability.

They are both quite different as its an internal DSL vs an external one. Just give them a try. A major difference at the minute might be package availability. Nix has more packages but some are broken, e.g. Julia. Guix has very clean and reproducible packages, and a slightly better CLI interface.