My vote is with the 12k. 100%
It just seems to me to be a better shave. I don't much care for comparing the speed or usability of the stones as what matters to me is the quality of the shave, which for me definately goes to the chinese stone.
Yes it is slow, and it is a huge stone(which I don't like, I wish I could cut it into a smaller piece) but for my face it works wonders. It just feels so much crisper(if that means anything)

I spent over 6 months using the coticule, which was great but I haven't used it since I got the 12k. I might use it if I wanted an inbetween stage, but I have a thuringen in a better size than a coticule so I use that instead now.

I would only buy another coticule if the price dropped significantly - which I don't see happening.
Don't forget though that much of this is down to preference and for you the coticule may well be better than the 12k. My thoughts are that if you have the 12k and are happy then the coticule is unneccesary but maybe it will be more to your taste(if you need a faster stone for example or if you want a small stone) You just need to justify the cost then.