The Onyx and Jade Platinums sound very, very similar. So much so, that I think the minor differences I hear in my system are just as likely due to unit-to-unit variations that the actual stone material (e.g. some Koetsu cantilevers are angled to ride lower than others). The Jade is slightly airier and the Onyx is slightly warmer and smoother, for what it's worth -- but again, this is only a slight flavor difference, as these 2 carts sound VERY much alike. My preference between the 2 has changed, and continues to change over time.
I used to run these on a Fidelity Research FR64fx, and these carts match equally well (that is to say, very well) with my Supreme II.
I've used them with a Lundal SUT, Cinemag 1131 SUT, and active JFET gain (Sonic Frontiers Phono 1) -- the Koetsus match excellently with any of these (50-100 ohms load if active gain).
If you really dig the look of the Jade then go for it -- but it is a "special order" stone, whereas Onyx is supposedly stocked. I have no experience with the diamond cantilever.