I really like Discourse. But having looked into it quite seriously, it has a lot of gotchas, drawbacks, and limitations. The interface is very nice, but the feature set isn't as "mature" as other older forum software. It seems like if you're using it exactly the way they use it on the official demo/Discoure site, then you're good, if you need to use it in any other way then tough luck.

PHPBB is fine if you skin it well. Plus PHPBB's system requirements are extremely easy to meet (PHP and MySQL). Discourse requires a more niche tech' stack and requires a much more powerful machine to run. Good luck running it on a $10/month VM.