We have several sonos boxes and an Echo. The Echo is very cool, but the sound quality is less than the Sonos Play3 or higher (maybe similar to Sonos Play1). We actually have both in the kitchen. Although, you can get the non-speaker version now and hook it up to your own bluetooth speakers. Sonos can play the same music to one or several speakers at the same time, easily. Echo cannot. I'm not sure if the Echo can play local music - I've never tried. I know the Sonos can do this easily.
Advantages of the Sonos are expandability, a very usable app, and multi-speaker support. For me, playing the same music in multiple room easily is the key point. The app is pretty easy to use.
Advantages of the Echo are voice commands, news reports, ask it questions, use it for kitchen timers, etc. The voice control is surprisingly good.