Dear David: I own the RSP and the 90X, both are different cartridges.
The Koetsu is really fine only if it is matched with the right tonearm: SAEC, Ikeda, Lustre, etc.., if not the Koetsu is a serious faulty ( at the frequency extremes ) cartridge. This cartridge really shine on those tonearms. Now, in the common today tonearms it is a " midrange frequency " cartridge but like you and every one that likes music know: the music is not only " midrange ", it is the whole frequency event.
The 90X is a top performer in every single area and I agree about with Vik and Kurt. This cartridge is much much better that what represent its " low price ".
About the ZYX any one that I heard ( till today ) is faulty too at the xtremes in different way than the Koetsu: not recomended if you appreciate the live music.
+++++ " It is also fairly easy to drive (for a MC cartridge anyway), given the 0.6 mv output. " +++++
This characteristic of the 90X is very welcomed along with a very user friendly in the set-up.. This Shelter is not only Stereophile class A but is too the reference cartridge for R. Harley editor in chief of TAS.
Yes, you are right: +++++ " I know its an old question and one thats impossible to answer, as cartridge choice is so personal.... " +++++
Now, there are other alternatives out there like: Allaerts, Transfiguration, Lyra, Ortofon, Van denHul, Sumiko, Denon, Benz Micro, Dynavector, Clearaudio, etc, etc.
Regards and enjoy the music.