Flarum looks very basic compared to Discourse. I’ve said it a million times before, making a forum platform is easy - making a good one is much more difficult and time consuming 
I mentioned in another thread that Discourse is my favourite forum platform now and I have used many different forum platforms over the last 15 years on various different kinds of community sites. However, I also mentioned that it doesn’t really matter what I (or anyone else) thinks - only you can decide what’s best for your community and you can only do that by spending some time with each of your shortlisted platforms. Better still, put up copies of your shortlisted platforms and get their feedback.
I’m pretty sure that if anyone spends quality time with various different platforms Discourse would come out top every time. But that’s just my opinion, what do I know 