I haven't tried any serious listening tests but the Apple Atmos streams sound pretty dang good to me. For music I really love I'll buy a hard copy in any case.
I have made critical comparisons using the reference album by Steven Wilson - The Future Bites, played from the BluRay release (full Dolby TrueHD Atmos) vs the same tracks on Apple Music (Dolby Digital+ Atmos) via an Apple TV 4K.
Comparisons were made on my dedicated home theater (5.4.4) and via a media room system (treated room) with 5.4.2 and via the Smyth Realiser A16 headphone processor.
First, no contest, the BluRay wins every time with greater dynamics & resolution, and to my ears, more positioning accuracy of objects.
That said, the Apple Music streams were not bad at all and sound great if you do not directly compare to a Bluray version. For much of the new music being released these days, I sure appreciate the opportunity to listen to it in immersive audio, and for most of it, that is all I need.
Sal, I agree, if it is very good stuff, I'll order the higher-res media.