Well the differences in Koetsu builds are always shrouded in a bit of mystery. I don’t have any experience with any Urushis or the Black, but I do have experience with the RSP and a number of stone bodies. The differences in stones like Jade vs. Onyx are pretty subtle and hard to even describe. The magnet type (Platinum vs. Alnico - I have the Alnico from an older Onyx Gold/Signature I had rebuilt) has a much bigger sonic impact. The Alnico magnet also has double the output level. The cantilever, diamond vs boron also has a much bigger sonic impact - the DC increases detail, clarity and dynamics without any obvious downside. The RSP (it has a clear lacquer, which I believe is missing from the lower Rosewood models) has less bass impact and power vs. the stone-body platinums, but the magnitude of this body difference’s impact is certainly less than that of magnet type or cantilever. And then you have the top-range Coralstone and Blue Lace stone models, which even with boron cantilevers seem to be a little "better" than the lower-ranked stones somehow, but it’s hard to say if this is due to being granted the "best" hand selected motors, and/or possibly getting slightly larger magnets (they seem a little bit higher in output, too) - no idea for sure, though.
In picking Urushis, the big choice is Vermillion vs. anything else. Again, I’d advise a proper SUT for the Vermillion (I like SUT’s with all Koetsus anyways, but that 0.2mV output will be especially tough without a SUT - and the Hashimoto HM-7 are fantastic with Koetsu, highly recommended). If you choose the non-Vermillion Urushis, just pick a look that pleases you! I think all the Urushis are beauties, and I'd love to try one someday. Probably would not be a Vermillion though, because even with a good SUT noise floor can creep in at that level.
