Koetsu Blue Lace Onyx vs. Coralstone

Suffice to say that I agree with you on cost. The Koetsu stone-bodied cartridge prices are particularly ridiculous, but so too are the prices of many many other "high end" MC cartridges. I purchased my Urushi in Tokyo for, lets say, much less than the US price. This year, too, I got involved in the thread here on vintage MM and MI cartridges, and I have bought several samples to audition. A select few of those are shockingly good, regardless of the fact that they are also relatively cheap. Of course, this is coming from someone who is declasse' enough to enjoy the Urushi.

I heard the A90 vs Puritas, each in a Reed tonearm, at RMAF, in several listening sessions over 2 days. (I kept going back to that room, because I enjoyed the presentation so much.) I assume that both were up and running for 8 to 10 hours per day for at least a few days before I got there. Dunno whether that constitutes adequate break-in for an A90, or indeed whether both had been in use prior to the show.

I found the a90 needs a good 100+ hours.

Listening in your own system is the only way to really tell how good something sounds - from my experience - and that's not possible with cartridges, of course,
unless you have friends who have what you're thinking of buying ....and they like you ;)

I'm a serious a90 convert. It's so instantly "dead-right" right out of the box even though it takes time to settle in fully. By "dead right" I mean it's just so obviously "there" (what??).

Oh dear ...words words words...

Ears know nothing about words, that's why I like mine so much.

.... the sound of the Koetsu Blue Lace Onyx is salubrious, exquisite, stunning, glorious and dazzling and the sound of the Koetsu Coralstone is superlative, smashing, magnificent, superb and impeccable...

Syntax - if you have to choose one from the two Koetsu wich one would you choose?

From my experience diamont cantilever takes these stone bodies to another level of performance (and price too).
I auditioned Jade DC and Coralstone DC as well as non DC versions.

Well, honestly, can't decide. They are both great ones. What amazes me all the time is that they are so much better than the wood bodies, it is hard to believe. A total different chapter from sonics. And both demand a serious metal Arm which has excellent energy transfer. When such one is used, they really come to life. When the System can do it, they sound like the real thing, not only with voices, with older classical records they create a holographic soundstage which is really unique. By far the best I ever listened to in the last 15 years. I compared them to the Lyras because I thought, the Lyras have probably a better detail but side by side they showed me that nothing is missing. Good as they are, but both, Blue Lace and Coralstone serve easily the same information AND more. Seriously a more of "being part of the recording", in a way, a superior view into the room (Vienna Sofiensaal for example)...in a way, I can FEEL the Ambience. Staggering.
Did not go for Diamond Cantilever, in a way I do not trust them, maybe others think different, but I prefer in a way "elastic" cantilever. Otherwise I use my cantileverless Ikeda System.