Having built apps in both, my view is that if you want the tool that fits best with Node.js' conventions, Electron is closer to that philosophy. I say this because of the way that the Node.js contexts of the main (backend) process and the renderer (front-end) process are kept separate. This is different to the way that NW.js has managed to blend the Javascript contexts of both into one, which can lead to odd behaviour when it comes to apps that run with multiple windows.
In terms of momentum, Electron is definitely capturing the most attention and it looks like it will eventually become the goto framework for cross-platform desktop applications over NW.js, which is a shame because NW.js has been around for a while now and was the pioneer of being able to build desktop applications with Node.js.
