illumos vs FreeBSD

Illumos is completely free and open source.

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.

illumos uses DTrace for performance analysis and troubleshooting.

Ports provide a wide collection of software which are easy to build, install and modify. They contain recipes and patches to build various software, so you can simply run "make && make install" to build and install the software. You may also keep local patches and it would be picked up automatically. It also allows you to use the latest software even if you are not using the latest version of the operating system.

Base system contains basic developer tools, including compiler, debugger and system utilities.

Jails allow for partitioning a system into several independent "mini-systems", a feature not found in many of the popular OS choices.

FreeBSD won't change everything from a version to another, it tries to keep and maintain old tools as long as possible, and won't replace half the system every two versions like GNU/Linux distributions such as Ubuntu, Arch or Fedora does. everything is stable and also pretty minimal. FreeBSD is designed for performance.