I was on Spotify for a few years then moved to Deezer because the UI was better, and it had better integration to the car system.
Having then gone back to Spotify about a year ago I reckon that the two are now pretty much inseparable for nearly all features. The UI’s are now pretty similar, integration with Sonos is equal, both offer download and play offline, streaming rate is the same and good enough for me, etc.
The list of compilation albums on Spotify is different to Deezer and definitely inferior. I really like the radio and Discover Daily and Discover Weekly playlists on Spotify which more than make up for the lack of the Deezer Flow feature. One thing I noticed about Flow is that it is not based on what you play, its based on favourites.
A key thing for me now is I have a new car with Apple Car Play which has direct support for Spotify but not Deezer