When comparing GNU/FreeBSD vs illumos, the Slant community recommends illumos for most people. In the question“What are the best server OSes?” illumos is ranked 7th while GNU/FreeBSD is ranked 9th. The most important reason people chose illumos is:
Illumos is completely free and open source.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Very customizable
It's more barebones than Linux so you can customize it every way you like.
Pro Not tied to systemd
Pro Clean design
Clang toolchain has a cleaner design than gcc, it's easier to implement a front-end to LLVM.
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...
Pro Zones
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.
Cons
Con Linux can do pretty much everything it can, and better
Unless you're looking for some niche server functionality, it's pretty much certain that a linux distro already has whatever you want in a more efficient and easier to use form.
Con Server-oriented, good for developing back-end software and not much else
Con No systemd
Systemd makes it trivial to automatically start and manage programs, offering a whole lot more control than a cobbled together bundle of some archaic init system, cron and random bash scripts.