I’m on NixOS and had the same feeling when first debated which to use.
If I was running some security sensitive app or scientific computing I would probably choose guix and they tend to be pretty rigid about reproducible building from source and signing changes to to the OS. But my day to day OS I use NixOS as I get tons of packages and basically all the same features with the trade-off of expecting some weird behavior on edge-cases and reproducibility (which I have yet to run into but know I will eventually)
If I was running some security sensitive app or scientific computing I would probably choose guix and they tend to be pretty rigid about reproducible building from source and signing changes to to the OS. But my day to day OS I use NixOS as I get tons of packages and basically all the same features with the trade-off of expecting some weird behavior on edge-cases and reproducibility (which I have yet to run into but know I will eventually)