I've been using Spotify (family subscription) for years. I recently took up a 4 month trial of Amazon Music out of interest to try HD through a Wiim Pro into the internal DAC on my PM6006, so we are talking fairly entry level hifi, really.
Did I find a difference in sound quality between the 2? Probably. Do I find it significant? Not really and it could just be imaginary on my part. HD has to be better, n'est ce pas? With better equipment and ears, your mileage may vary.
To be fair much of my Spotify listening is done on the hoof through bluetooth headphones where the difference is effectively none.
The biggest difference is the interface. Spotify is good, Amazon I dislike.
I doubt I'll keep Amazon beyond October but I'll do a bit more comparison between the 2. Personally, I really prefer physical media and all I need from streaming is on-the-go audio and some music discovery. I also doubt I'd pony up for the hifi version of Spotify when it does appear.
I did try YouTube Music on a 3 month trial, and did not like it at all. I felt it sounded inferior to Spotify.