CMK,
What's the tonearm and phono stage you plan to use? Koetsu is never lacking in 'audiophile' quality, but what it can bring, further, is a 'magical' musicality so hard to duplicate by other cartridges. I have the 'other cartridge' set up, but in direct comparison in my system, anybody would've pick the Koetsu, as music just flows out. It is pointless to worry and listen to 'audiophile quality' when the music just mesmerizes.
Urushi is quite a bit better than RS, in fact, I prefer it over Onyx.
The reason I mentioned phono stage and tonearm because Koetsu seems to work with most tonearm, very rarely there is any compatibility, but can't say that to other cartridge. Regardless of using RB300 or SMEV, the Koetsu always sounds like a Koetsu. However, phono stage is very important, as it could very well be a limiting factor, if you have the better cartridge, but not a good performing phono, the upgrade effort is not worth while.
I am not affiliated with Koetsu, but own/owned various Koetsu, and other cartridges over the years. My ultimate cartridge purchase would be an Urushi Platinum.