I have not heard Anna either but heard A90 a few times and had Koetsu Onyx in my system for a little while. Like Albert said, the sound of both are so different that you really need to hear both. Koetsu is one of those either you love them or hate them and is rather picky about the tonearm. I was rather underwhelmed when I heard Onyx with JMW 10.5 and TW Acoustic Raven arm. Coralstone also was not so impressive with Reed 3Q but with FR64s, it was a different story. I have not heard Koetsu with SME V so I don't know how well they match. With the right setup and the right kind of music, Koetsu is rather hard to beat but I don't consider it a super great all around cartridge for all kind of music though. So much will also depends on the type of music that you play as well. Personally, I would pick Koetsu over A90 if I have the right arm for Koetsu just because of my listening preference and the type of music I listen to.
If financially feasible, the best thing to do is to try both and decide for yourself.
If financially feasible, the best thing to do is to try both and decide for yourself.