I have both a coticule (from Howard), and a Shapton 15k for finishing stones. I like them both very much. The difference as far as I can tell is that the synthetic stones hone much faster and I have to be careful of overhoning. I love my coticule because I have never overhoned on it and it gives me a great shaving razor. I don't use a slurry on mine either, and this allows me to take lots of passes (say 20-25 passes for a touch up after every 10-15 shaves).

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