When comparing XNU Darwin (macOS) vs illumos, the Slant community recommends illumos for most people. In the question“What are the best open-source Unix like operating systems?” illumos is ranked 4th while XNU Darwin (macOS) is ranked 8th. The most important reason people chose illumos is:
Illumos is completely free and open source.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
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 Not open source
macOS itself is not open source, but its kernel is.