When comparing SteamOS vs NixOS, the Slant community recommends NixOS for most people. In the question“What are the best Linux distributions for desktops?” NixOS is ranked 19th while SteamOS is ranked 79th. The most important reason people chose NixOS is:
Atomic non-destructive upgrades / rollback of a system upgrade / declarative reproducible system configuration / unprivileged installation of packages / transparent source or binary deployment.
Specs
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Pros
Pro Native graphics processor support
The OS natively supports Nvidia, Intel, and AMD graphics processors.
Pro Streaming service integration (under development)
Valve is in talks with streaming companies such as Spotify and Netflix to bring their features to SteamOS.
Pro For entertainment, it just works
What media center software do for videos, it does for games (and later videos).
Pro State of the art package manager
Atomic non-destructive upgrades / rollback of a system upgrade / declarative reproducible system configuration / unprivileged installation of packages / transparent source or binary deployment.
Pro Minimal
You can start with a minimal environment and add packages and software to suit your needs as you go along.
Pro Reproducible system
NixOS is configured using the Nix package manager, allowing your system to be replicated and kept in sync across multiple machines. Great for keeping a laptop and desktop in sync.
Pro Robust
Packages don't break after a NixOS upgrade as they are prone to with other distros (especially Arch).
Cons
Con Hard to install
If you've installed Arch you're probably fine, but for those used to simple installs like (Ubuntu to name just one) you should prepare some patience, determination, and painkillers. Good news is, it's getting better (it's very young).
Con Not built around the desktop experience
SteamOS is, first and foremost, meant to act as a gaming console - it doesn't ship with even some of the most basic applications, such as a file manager or image viewer. As a result, using SteamOS as your primary operating system would require a fair bit of work.
Con Documentation is not good
A lot of the documentation of various functions is buried on the source code, their respective manuals, or non-existent. The documentation, the conventions, and the scattered toolchain really made searching for stuff easily missable.