People forget one very important factor.
Yes, Discourse requires a bit of resource to run initially, but it can also handle quite a big activity load before it requires any additional resources on top of the ones it took when initiating the app to begin with.
If a forum has 1 million active users, will Flarum take less resources then Discourse? Based on it being buildt on PHP, highly unlikely, it's almost guaranteed to consume more resources then Discourse.
Discourse is a board intended for high-end communities with a lot of activity, it's not without reason that some of the biggest tech companies have chosen Discourse, if they ever move to another platform, it's likely never going to be Flarum or any other PHP driven board. Discourse use-case is not for non-programmers, dev-ops and so on, it's intended to be installed, setup and controlled by those exact people, it's not Wordpress, never has been, never will be. Unfortuantly, then Flarum is taking this path as well, and i would strongly discourage any non-it-personal to even attempt running a Flarum board, a single issue and your in deep shit.
All in all, in my opinion, non is better then the other pure design / functionality wise. The real benefit comes in when you mention visitors, activity level and use case. Flarum and Discourse is two boards that does the same, but they are intended for very different groups of people.