When comparing illumos vs GNU/Linux, the Slant community recommends GNU/Linux for most people. In the question“What are the best server OSes?” GNU/Linux is ranked 1st while illumos is ranked 5th.
Ranked in these QuestionsQuestion Ranking
Pro Open Source
Illumos is completely free and open source.
Pro ZFS file system
illumos uses ZFS as its file system. ZFS is also a logical volume manager besides being a combines file system.
The main features of ZFS include: protection against data corruption, data compression, checking continuous integrity, high storage capacities support etc...
Zones provide an isolated environment to run applications in. Processes that run in zones are prohibited to interfere or monitor any other activity in the system.
Pro Performance analysis and troubleshooting
illumos uses DTrace for performance analysis and troubleshooting.
Pro Access to really powerful terminals
Pro Lots of development tools available
Pro Most likely also your deployment target
Makes testing while developing easier. According to a September 2014 study by W3 Techs, *nix based servers are used on over 2/3 of websites.
Pro Most software is open source
Pro Large percentage of Linux users are developers
Pro Package managers
You can install any library or package that you need (gcc, php, node) with just a couple of commands in the terminal.
Pro Most Linux distributions are free
Pro A wide variety of distributions available
With a lot of variety, one can use the distribution that fits the type of work best because of the many choices that are given, instead of just one.
Pro Follows the UNIX philosophy
The UNIX philosophy: 'Write programs that do one thing and do it well'. Since Linux itself follows this philosophy then it's very easy to start creating scripts and programs.
Pro Familiarity with Linux is often required from a developer
Many university computer science programs are based on Linux and in any case, you will inevitably be dealing with a Linux box of one flavor or another someday, be it a server (most likely) or a workstation. The languages and methods used in the Linux/Unix environment (e.g., bash, C, C++, Make, etc.) are very commonplace among developers and are to the computer side of the discipline what the English language is to the human side of it: the common language.
Nice, developer-friendly environment.
GNU/Linux handles desktop sessions differently than Windows. Users may customize their own sessions; in fact, a single user may use different desktop environments for different login sessions.
Pro Choose any type of desktop environment (or none)
Most Linux distributions support a range of desktop environments, be it plain old X, a tiling window manager or a fully fledged mammoth desktop like GNOME or KDE.
Pro Works great on older hardware
7-10-year-old Dell laptops can run Unix or Unix-like OSes very well, where Windows would grind/drag/vomit.
Pro Hardly ever crashes
Pro No telemetry, unlike Windows
Pro Lower chance of data loss
Linux has very few viruses. So there's almost no chance of getting infected by a virus and thus losing your data including your important programming files.
Pro Extremely fast
Can be made even faster by going GUI-free or using a lightweight window manager.
Pro Just works
Linux tends to just work with little to no effort or troubleshooting required.
Con Issues with drivers if your hardware is not officially supported
Con Maintenance can be time-consuming
Con Steep learning curve
Con Too much customization
To get features on par with OS X, you need to research packages, install them and configure them. Even then, it may not be as good as OS X.
Con A wide variety of distributions available
With a lot of variety, one cannot deploy to a single system and has to prepare for a bundle of distributions, instead of just one.
Con Less and worse professional software is developed, due to the low user base
Depending on what type of work you are doing, you may find Linux software lacking compared to their Win/Mac counterparts.
For example in game development, tools, like Unreal Engine or Unity, usually lack in quality or novelty compared with Windows. Having crashes or bugs that aren't fixed for a while.