OK, but I still want an answer to my question -- to help me decide what distro to choose for personal use.
(I already get that the design of NixOS prevents the system's ending up in an incoherent state, which will happen on Arch eventually if you wait long enough between upgrades.)
I've developed packages for ABS before, and it's nice. What nix's model gets you is (a) largely avoiding complex bash scripts, and (b) a pile of tools for working with packages that you don't want to install semi-irreversibly. nix-shell, for example, will put you in a shell inside the build environment, and you can run the build steps yourself and see what the outcomes are. nix-build will build a piece of software and create a symlink to the output, but not install it into your user's package namespace.
One of the nice things is that you can install many things without having to sudo - the build is run by a daemon and sandboxed. nix-shell can also be used to create a shell in which a given package set is installed - you can use that to use a piece of software as a one-off or create a development environment that doesn't pollute your general system. Tools like home-manager[0] can help with managing your home directory in a similar way to NixOS's management of your system, too - I have redis and postgres installed using home-manager to run as my own user on demand under systemctl.
(I already get that the design of NixOS prevents the system's ending up in an incoherent state, which will happen on Arch eventually if you wait long enough between upgrades.)