I find Discourse very well documented and proven for many production use cases. Unless you are more familiar with PHP, you will find much more support on the Discourse community forums and the internet for it compared to Flarum. Flarum was just recently released for use in production. Though the last time I used it was when it was in beta, you might find that you need to be more hands-on when you run into any issues.

I advise that you use Discourse. I've not tested any Python based forum software.