Suehiro rika vs. 6k

[SpikeC]

I find myself in possession of a Suehiro 6K stone, and I am wondering what the functional difference is versus the 5K Rika.
The 5K seems to be the only one used by the knife knuts, and I'm wondering why.

 

[mainaman]

I never knew there was a 6k , not even sure where can I get one from.
On another hand I never liked the 5k.