When comparing Plan 9 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 7th while Plan 9 is ranked 11th.
Ranked in these QuestionsQuestion Ranking
Pro Well designed
Plan 9's design is very orthogonal and well done.
Pro Designed to allow for multiple computers to work as if they were one
With Plan 9, computers are turned into terminals where you can access files and processing units which can be located elsewhere. Data storage is handled by another external server. This means that instead of each person having their own individual computer with all the required parts, all of the computing and storage is done in a central area and everything is networked in together.
Pro Very Fast
Pro Beta has been released
After about 6 years since the alpha version, beta has been released on Fri, 2018-09-28. Check here for release notes.
Pro Only need 512mb ram
Con Difficult to use software made for other systems
Plan 9 is very different from most other operating systems, and as such it's extremely difficult to bring in software designed for other operating systems ('porting').
Con No fully featured web browser
As indicated on the official Bell Labs webpage, there is no full featured web browser currently working on Plan 9.
Con Not supported by most tools
Not supported by most dev tools beyond text editors.
Con No one uses it
It's a very niche OS that no one uses.
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.
It's still in beta and quite unstable. Making it unsuitable for developing applications of any kind.