My personal preference is NodeBB. However, Flarum has come a long way in a short space of time, but really needs to shake the "beta" branding to be taken more seriously. What I do notice is that Flarum has heightened interest in terms of the discussion form than NodeBB does, although that is by no means any indication to say one is more popular than the other in my view.
Madchatthew Is there certain applications that one is better for than the other?
There are some things that Flarum excels at, and there are several key areas of functionality where NodeBB clearly leads the race. NodeBB is way faster than Flarum - mostly because it is pre-compiled and does not use PHP at all. It also has superior functionality in the sense that it has a widget system, plus a decent way of installing plugiins that closely resembles WordPress.
Madchatthew Do you find one easier to setup than the other?
Both require CLI to setup, but once installed, NodeBB can be managed and upgraded (both core and plugins) from the web GUI, meaning that in most cases, you no longer need the CLI
Most of the security in Flarum is controlled by NGINX or .htaccess configuration, whereas NodeBB has a page in the web console dedicated to this. In terms of features, NodeBB excels, and leaves Flarum totally in the dark. For example, Web Socket support, Chat, and several others are provided out of the box, meaning zero cost. Flarum has something of an ecosystem for extensions, but it is nowhere near as comprehensive as NodeBB.
As you know, Metabullet runs Flarum, and Phenomlab runs NodeBB. Look and feel-wise, Phenomlab is the clear winner in terms of speed. Here's a good example of a speed test
As you can see, Phenomlab.com is way faster. Not so much in the overall speed, but the delivery of content. For example, Metabullet.com is tuned to deliver on what is critical for the platform to run, so 21 requests. Phenomlab.com in fact carries much more functionality, hence the 82 requests, yet is 50% faster than Flarum. Both of these sites run on the same server, based out of Germany (hence the location in the test). The TTFB (Time To First Byte) clearly speaks for itself. If you measure the "waterfall" between Metabullet.com and Phenomlab.com, the results are quite astonishing. The real reason for this is that PHP is not involved, which invariably makes for a much better experience in terms of speed.
Finally, NodeBB has it's own caching engine, which in my view, is by far superior that what Flarum offers.
Overall, I much prefer NodeBB because of it's extensibility, flexibility, and overall ability to become whatever you want it to be with little effort - and no need to keep making changes to the system to get desired functionality.