Interesting. I have both, in several of the same rooms, and I find that I use Sonos all the time and almost never use Alexa. But I think I'm the oddball in the world, because I find voice control (I have both Alexa and Siri in my life) to be annoying and clumsy most of the time. I'd rather just tap or type. And the sound quality on Sonos is so much better that aside from voice control, Alexa seems to have no advantage. But again, I know I'm the weird one here - everyone else seems to love voice control.
Ah, is this the reason behind Sonos' conspicuous exclusion from the list of AirPlay 2 partners at WWDC? It was a hellstorm on Twitter after the fact, with plenty of Sonos faithful begging the brand for an answer...
It seems (seemed?) like Apple and Sonos happily shared some consumer overlap until then. I find this news quite intriguing in that light.