Same question here. Seems like the main advantage of yarn is their caching mechanism and flat dependency system. npm 5 appears to use some sort of caching (projects now include package-lock.json), and is supposedly much faster than before. I wonder if yarn still has advantages over npm?
The answer is simple: any of them will work just fine. npm 5 brings many improvements, a lot in performance (in my tests the speed between them are very similar), and new functionalities (e.g. outdated command).
Upvoting because it is currently a very confusing ecosystem, and I'm not sure of the right answer.