I have used both Pico and Nano. The selling point of Nano to me is it being small and available. I don't do my day to day work in nano, but even in the most minimal system it is normally there, which makes it useful for configuration changes in a pinch.
Another editor that works similarly to Nano, that most people don't seem to know about, is Micro (GNU) Emacs. Although it's not as small or common.