The focus is on the theory and principles, and not as much the implementation, correct? If that's the case, use whatever is most comfortable for you as the instructor, so you can focus on the concepts instead of whether or not you're using such-and-such framework or language correctly.
I'm a Java and Kotlin coder, mainly using Spring. I could whip out a reasonable demo API in ten minutes with those and dig in on the why's and how's of REST from there. If I'd asked your question and someone said I should do it in Express, I'd be so worried about whether I was giving them a good representation of how to do it in a new environment that I'd bomb everything else in the lecture.
The important thing is to get the concepts across well enough for them to understand. Then they can apply those in whatever language and framework they need or want to use.
Or bonus: you said you're comfortable in both Rails and Express (and the Elixir one). Use all of them. Then you can more effectively demonstrate that the concepts don't necessarily apply to a single framework or language, and they just need to find out how to accomplish what they want when they pick up something new. (By the time their careers really get under way, there'll be some other new hotness that will make everyone sneer at today's flavor.)