Of those two (nano/vim), I've also found it easier to introduce people to nano. I personally use vim, but if I'm doing a tutorial for people who aren't very familiar with the unix terminal, and at some point in the tutorial they need to edit a config file, I'd rather not take a big digression into explaining vi's modal-editing concept just to change one line in a config file.
You're assuming Linux. BSD distributions don't install nano by default. vi is a mandatory POSIX utility, nano is not.
This doesn't help the problem of knowing what to type on any system up-front, but fwiw, FreeBSD ships with a nano-like editor in the base install, ee(1).
Are there better, more powerful editors out there? Sure. But just like vim, you can expect to find nano pretty much anywhere, and if you need to make a quick change, say to a configuration file on a bare-bones system, that's nice.