Adam seems to have a bit more of a preference towards softer stones. Michiel prefers stones a bit harder - Shaptons and, among the Naniwa stones, the 8k snow white. I find the 12k Superstone harder than the 10k. Under the scope the 12k SS does produce a finer finish, but the softness of the 10k SS does give the perception of greater refinement. These are all very valid and reasonable perspectives. The 10k and 12k SS are popular for straight razor honing as well. My preference here for straights is the 15k pro.
For a straight razor, I certainly prefer greater refinement. Shaving with a final 8k edge is IMO barbaric
The 15k Bamboo is not as hard as the Shapton 15k and it appears to be a bit more refined, the best of both worlds, and would be my ultimate choice. But given your choice of a 12k SS, it's time to move on to compounds. So 12k is approximately a 1.5 micron edge. I'd begin by doubling it to a 0.75 micron (24k) CBN on Balsa or nanocloth. I prefer nanocloth over balsa. Following that with a quarter micron (64K) CBN on nanocloth and then a tenth micron (160k) CBN emulsion on Kangaroo as an ideal sequence. Later on as you grow accustomed to these extreme edges, you can go to a final level of refinement of 0.025 micron Polycrystalline diamond.
For finishes as coarse as quarter micron or coarser, diamond tends to produce a less comfortable edge. For grits tenth micron or finer, this effect seems to 'go away'. This is understandable, since an 0.025 micron polycrystalline diamond finish is a scratch pattern 10x finer than a quarter micron finish. For the intermediate range, I prefer CBN both for cost and effect. CBN is more expensive than monocrystalline but less expensive than polycrystalline diamond. Polycrystalline presents multiple sharp edges on a particle the same size as a monocrystalline particle so it is effectively a finer scratch pattern and because it has more sharp edges it also cuts more aggressively. For embedded applications, monocrystalline has advantages in that the particles maintain their size and structure and stay embedded longer in the typically nickel or resin like matrix holding the particles in place.