The Audio Aero Capitole II is slightly better than the Resolution Audio Opus 21, sonically speaking, IMHO.
(Not that the Opus 21 is anything less than a great cdp, as I will attest to as a proud owner! Until I heard the Capitole II, it was the best I had heard. It sounds as close to vinyl as I had heard, .... until I heard the Capitole II.)
However, I have heard them both and I agree that the Capitole II is slightly better sonically. It sounds just a bit better, and a bit closer to the sound of vinyl.
(As you can see, I compare both to vinyl, as the sound of vinyl still reigns supreme, in my book anyway! A good analog system will beat both of the aforementioned CDPs, and not by a small margin either. My opinion of course!)
Now, whether the Capitole II is worth 2 to 3 times the cost of the Opus 21, is another matter. (The cost of the Capitole II is about $8K new/$4,000 used, assuming you can get good deals, and the Opus 21 is $3,500 new(never a discount as it is sold direct from the manufacturer!)/and $2,500 used).
For me, it is not worth the price difference, as I would much rather put all my extra cash into my analog system, and the Opus 21, while not quite up to the Capitole II standards, is close enough for me!)
However, if you can afford the extra money, and you really want the best, go for the Capitole II, especially if you don't have an analog source!
My two cents worth anyway.