I never used yarn, but I just did some research about the changes in comparison to yarn.
Well, actually they are more similar now, obviously (saving by default for example). In terms of speed, npm has improved as well (https://twitter.com/passcod/status/860437637764571140). Even though npm still seems to be slower than yarn (https://docs.google.com/presentation/d/1re-AGrTOf4zmnXXDMTG1...). At the end, I'd say it's about personal preference, they both get the job done.