I'm told the more recent Coticules are inferior to the older ones and even those came in grades so it depends whether you have an OK one or a good one or a very good one or an excellent one.
The one I have is probably about 15 years old and when I got it that Coticule was my best stone and I used it as a finisher and I got great edges from it. Of course when I got my Escher the edges were better but the Coticule was darn good for shaving.
Given that they are all in excess of 70 million years old - or 700 hundred million depending on which book you read, the fact that you can discern a difference in 15 years is incredible.