As you points out both are open source, we can host it where fits our budget with expected performance.
In my knowledge, node/javaScript should performance better than ruby (in general, given that both run the same algorithm on the same hardware)
At the same time, all the NodeBB sites and all the Discourse sites that I visit provide good speed. (although NodeBB sites seems a bit faster without any insights of their servers).
On the other hand, Discourse sites give me the impression of a lot of plugins/themes available, which should set me up for a good start.
I asked Devs about the plugins situation to learn more ecosystem of Discourse, and how it is “better” than NodeBB to help me finalize my choice.