When comparing macOS vs Plan 9, the Slant community recommends macOS for most people. In the question“What is the best operating system for a developer?” macOS is ranked 2nd while Plan 9 is ranked 7th. The most important reason people chose macOS is:
The UI of Mac OS is rather unrivaled. The smooth, responsive, and cohesive UI makes the system quite joyous to use.
Specs
Question Ranking
Pros
Pro Polished UI
The UI of Mac OS is rather unrivaled. The smooth, responsive, and cohesive UI makes the system quite joyous to use.
Pro Based on Unix
macOS being a UNIX certified system means that you can install a lot more stuff with a lot fewer headaches than if you were on Windows.
Pro Powerful terminal
It's very similar to a Linux terminal.
Pro More commercial software and gaming support compared to other Unix systems
Adobe CC, MS Office, Steam games.
Pro Best support for Objective-C
Pro Easy access to lots of great dev tools
There's a large selection of great development tools available for OSX. The operating system itself comes bundled with a powerful terminal emulator, called Terminal. Additionally, Apple provides tools, like Xcode, an IDE that contains a comprehensive collection of tools for developing OSX and iOS software, for free.
Pro Ideal setup, out of the box
Next to no custom configuration is necessary.
Pro Lots of open-source software available
Because it's Unix under the fancy GUI, most open source ports easily to it.
Pro Has many special tools for developers
Has support for multiple IDEs
Pro Great Git GUI tools
Tower, Kaleidoscope, SourceTree
Pro Has software that only runs on Mac
For example, Sketch.
Pro Streamlined workflow between devices
Because this is an Apple product, there is a streamlined workflow between your computer and all mobile devices. For example, if you type an a Pages document, once you save, you can open the updated document just moments later on your iPad, and vice versa. The same goes for iMessage, (yes, you can text people with your phone number from your computer. Actually, you can text other people with apple devices with just your Apple ID, with or without a phone number, for free!) Numbers, Notes, Reminders, Contacts, and just about any other Apple workflow application.
Pro Great modifier key layout
Pro Using VMware you can also run Windows 10 on the Mac
This is useful for testing and some development tools that are Windows-only (XML Spy, MapForce).
Pro You need it to compile macOS or iOS apps
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 Well designed
Plan 9's design is very orthogonal and well done.
Cons
Con Expensive
OSX is tied Apple hardware and Apple hardware tends to be expensive.
Con Limited hardware
Usually, the hardware that can run this can't be upgraded.
Con Most software is closed source
For people who like to use open source tools for their development work, this may be a problem. There's plenty of advantages to open source software, one of which is the ability to tinker with and customize the tools themselves that you are using. Although there's plenty of FOSS tools available for Mac, especially through Homebrew, the number of packages available is much lower than the number of packages available for any Linux distribution.
Con No native package management
A comparison of package managers available for OSX can be found here.
Con Very few options for running hosted, on the cloud
Con Closed source
Mac OS is closed source itself, which means that it is developed more slowly and has more problems
Con Bash version is obsolete
macOS comes with an obsolete version of Bash, due to licensing issues.
Con Poor application support
Fewer apps run on Mac OS than on Windows or Linux
Con Silly modifier keys layout
The Command key is strange, Alt is where Super should be.
Con Too little customization options
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 Not supported by most tools
Not supported by most dev tools beyond text editors.
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.