Both features are important IMO, reproducibility is for being able to define certain aspects of your machine in a way that you can nuke it and, as long as you have its configuration (declarative for Nix, other implementations might have it as imperative), bring it back just how it was set up, without differences or breakages; while immutability is for being always confident that whatever* you do to your machine, you won’t be able to break it because the root, which holds the functioning core of your system, can’t be messed around with, NixOS has both I believe.
*not really “whatever”, because there are still some ways to break, but you have to be very deliberate in doing it (think rm -rf /*), but in normal operation you won’t just somehow install something or upgrade your packages and be left with an unusable system