When comparing NeXTStep / OpenStep vs Haiku, the Slant community recommends Haiku for most people. In the question“What is the best operating system for a developer?” Haiku is ranked 6th while NeXTStep / OpenStep is ranked 9th. The most important reason people chose Haiku is:
Originally Haiku was based on the BeOS which has since been discontinued. The alpha release was in 2009, and the most recent alpha build was compiled late 2012. There are still regular updates on the [official website](https://www.haiku-os.org/), and development hasn't stopped.
Ranked in these QuestionsQuestion Ranking
Pro Immense amount of history
NeXTStep has been part of many historic events. WorldWideWeb (later renamed to Nexus) - the first ever web browser was created using a NeXT computer. It also has a history shared by gaming - revolutionary games such as Woldenstein 3D and Doom were also created on NeXT machines.
Pro Development is supposedly going to begin again
Originally Haiku was based on the BeOS which has since been discontinued. The alpha release was in 2009, and the most recent alpha build was compiled late 2012. There are still regular updates on the official website, and development hasn't stopped.
After the sale to Apple (which used NeXTStep as a base for it's operating systems, including macOS, iOS, watchOS and tvOS), NeXTStep was discontinued. OpenStep doesn't seem to have had any significant use since 1996 when it was used as a base for SPARC (scalable processor architecture).
Con Small community
It is important when developing to be familiar with tools that other developers use. You can make any utility in any language you feel like, but if it's in an esoteric language that no one can read targeting a small platform that no one uses, then it was just something you did as a hobbyist, not as a developer.
This is not to say that Haiku isn't a great operating system to hack around on. Just don't delude yourself into thinking you're doing it to get familiar with tools that you need to know to be a better developer.