When comparing Discourse vs Flarum, the Slant community recommends Flarum for most people. In the question“What are the best web forum software packages?” Flarum is ranked 1st while Discourse is ranked 2nd. The most important reason people chose Flarum is:
The extensibility allows you to install and enable only the extensions that you desire. This is beneficial to the performance. In addition it's a SPA so js and css are loaded once and retained.
Specs
Ranked in these QuestionsQuestion Ranking
Pros

Pro Has a built-in immune system from trolls, spammers and bad actors
Discourse has a simple user trust system that makes moderating the forum a lot easier. Users gain more permissions as they gain more trust, which limits the amount of damage spammers and trolls can do. Discourse co-founder Jeff Atwood also founded StackExchange, which is the gold standard for gamification/moderation systems so you can also expect Discourse to get better and better at moderation.

Pro 100% free and fully open source
Discourse is powered by Ruby on Rails, Ember.js, PostgreSQL and Redis. The code is licensed under GPL and available on GitHub.
Pro Modern & polished UI/UX with lots of great smaller features
The best feature is its design: discourse is designed to aid in the creation of high quality online conversations. Flat discussions with well implemented quoting systems, expandable/collapsible replies, infinite scroll, clean UI and many more features come together to form a highly polished forum experience that makes legacy forums show their age.
Pro Helpful community
Discourse has a comprehensive forum culture support hub at meta.discourse.org.
Pro Hosted and self-hosted solutions
You can host it yourself in a $5 cloud server or on any Linux server you already have. And if you don't want to deal with this, you can use the hosting platform from the development team.

Pro Live preview while editing the post
The post editor has a synchronized side-by-side preview of the Markdown rendering of the post.
Pro User selectable themes
Users can choose between the themes made avaliable on their instances, and even use different themes on different devices (mobile vs desktop, for example).

Pro Export your posts

Pro Effective search
Pro Good to go out of the box yet infinitely tweakable
Discourse has sane, safe out of the box defaults, but a million dials and knobs to tweak.

Pro Easy one-click upgrade
Discourse can upgrade itself with one click in the admin panel. Not the same can be said about MyBB, or most other forums.
Pro Advanced JavaScript app
Runs in modern browsers and works identically on desktop and tablet and smartphone without the need for a native app.

Pro Discussions can be organized in categories and tagged
Discussions can be tagged with an arbitrary number of tags, separately from categories. Categories are hierarchical and don't scale; tags do.

Pro Official Akismet Anti-Spam Plugin
Official Akismet plugin vets posts by new users to make sure they don’t look spammy before they hit your site. Akismet key purchase required.

Pro Extensible via plugins
Pro Good on-boarding experience for new users
New users are greeted withe the disco bot which has an interactive experience to teach the basics of using Discourse.
Pro Very fast loading time
The extensibility allows you to install and enable only the extensions that you desire. This is beneficial to the performance. In addition it's a SPA so js and css are loaded once and retained.
Pro Open source
Released under the very permissive MIT license, the source is freely available on GitHub and the Flarum Foundation was set up to guarantee the open source nature of Flarum as well as guarantee independence.
Pro Easily expandable
Most of the features are extensions, you have free control to en- or disable and remove extensions according to your needs. That keeps Flarum thin, lean but also extremely suitable for all types of communities.
Pro Modern UI
Designed to be mobile-first, while still allowing a rich interface on desktop. Flarum is created from the ground up to be intuitive and simple to work with.

Pro Great community
The community building flarum is fantastic and very helpful!
Pro Mobile Friendly UI
Pro Lots of features
The developers do a great job adding new bells and whistles.
Pro Based on (but not built with) Laravel
Flarum is based on many components from Laravel (like Eloquent) and Symfony, thus providing a sense of familiarity to developers with experience in those frameworks.
Pro Easy installation
As long as you have access to composer, installation is a breeze. This also makes extension management install/removal very easy as the two processes are one and the same.
Pro Intuitive
Very easy to use and feels very modern, unlike most of the competition out there.
Pro Easy installation on shared hosting services
Available on softaculous (an auto-installer for web hosting control panels like cPanel).
Pro Steadily growing
In the last few months Flarum is experiencing growth like never seen before, it's definitely time to jump on board.
Pro Runs on shared hosting
Easily runs on low end shared hosting hardware as long as you have SSH access for plugins. Also available on Softaculous for auto install which makes it a 1 click job.
Cons

Con Complicated setup
Compared to standard PHP+MySQL apps, Discourse is more complicated - the app lives in a Docker container. However, a Docker image is available, which reduces the setup time to about 30 minutes.
Con Leaves out users with shared hosting
Discourse requires sudo, which is only available on a VPS.
Con Bad noscript support
When using a Discourse forum without scripts the experience is greatly reduced. It's really hard to use and is read-only, meaning that one can't comment or create threads without scripts.
Con It is more "Free to Try" than really free
Discourse is open source free software. The hosting is where the costs come in. Discourse.org hosting starts at $100/mo, but you can pay as little as $5/mo on Digital Ocean or other cloud hosts.
Con Extremely expensive hosting
Official discourse.org hosting starts at $100/mo and supports the team creating the software. It can also be hosted on a cloud service like Digital Ocean for $5/mo.
Con Need composer
Unable to test Flarum because composer is not a standard in vps.
Con Extremely complicated to install
It's extremely hard to install Flarum, especially using anything that is hosted by Amazon. Good luck with a simple install for Amazon.
