Hi @roberto77
Thanks for your post!
No-one can tell you which service is better - it’s a personal thing, really.
If you find Deezer has more of the tracks you ask for, however, it would seem to be the best fit for you. Given that you are wanting to use Alexa and voice requests, I would presume that using the service’s own app would be less important? You can always browse the service through the Sonos app if need be.
In terms of received audio quality, Deezer wins out with lossless FLAC streaming.
You could try searching Google for “deezer spotify comparison” to see the opinions of others.
I hope this helps.