First, I've made my research of the differences between two (check out this great post). Then I made proof-of-concept in both projects, just to see what hiccups I need to expect.
Electron is actively developed and supported by Github, has a lot of good npm packages and if you look at it, made a lot of progress in just 2 years. I would say that NW.js is easier and friendlier for day-to-day development because Node context is available in renderer as well.
Both are good, and both have some caveats. Minimal app which I've rewritten in NW.js and Electron is over 100 Mb, and was only 1 Mb originally made in Adobe AIR. But I was aware of it before I started.
I decided to put focus on Electron. Unfortunately, there are not much technical resources and documentation is still under active development. I needed to check out the source and experiment a lot. I made some notes here and there, so I'll put them down here as well and hopefully help someone.