I quite agree with Milt, but I also understand Simon M statement/question, aren't they the same 1's and 0's? I can only tell you my experience. When I was new to high res my first experience was with single layer SHM SACD Japanese stereo. I found the titles offered and the quality of the listening experience way above others. Then I tried to enhance my listening experience and purchased many mini cardboard SHM CD's, one for the Japanese attention to detail of the mini albums and the enhanced listening experience. I first noticed this when I had the US RBCD box set of The Beatles, black box, I then became very intrigued with the possibility of getting the stereo Black Box Japanese SHM CD release and I purchased. I could absolutely tell the difference between the RBCD and the SHM CD. One thing that helps is one you notice the difference based on a lifetime of intimate knowledge with a recording, then things pop out a little more. If I don't know a recording that intimately I might be less to notice the difference. I will never discount placebo, I am paying more therefore it must be better=placebo. I do try to acknowledge placebo and I do ask myself is it really better?
The actual SHM CD's are a tiny bit thicker, this is very noticeable playing one in the car as they get a little stuck when ejecting, all of them are like that. Is the extra thickness good for a better optical read with laser?
There is also BluSpec CD, my copy of Hendrix Electric Ladyland definitely sounded better than the RBCD, but now with the 2018 Hi Res release both are obsolete.
I have not purchased any UHQCD's as the titles are to repetitive as I have most of them.
I will also be the first to say that if you can get a first generation RBCD (early 90's) of a certain recording it will likely sound better than any SHM CD.
I have no problem paying the extra money for any recording in the SHM CD format, the Japanese are stellar, bar none, to any other. How it is shipped, the condition it comes in, the extra details, slip covers, disc covers, extra liner notes are always superior. Am I right/wrong, I don't know, but I do know without doubt I feel my listening experience is enhanced with a Japanese release as I am the only one that counts as I sit and listen by myself.
Note on Japanese Abbey Road release: Perfect, no issues, great packaging, sounds great, BD works perfect, I ripped the 2.0 and 5.1 DTS HD using MKV and Music Media Helper, no problems, all perfect. Did I pay $25.00 more than most, sure, but I am happy.
Here is a nice Steve Miller Japanese SHM CD pre order I will purchase.
Book Of Dreams Pre Order
Thanks for letting me ramble and for continuing to educate me.