I don't know a lot about echers, mostly because they seem to be harder to find (at a price point I will bite at), but they are typically considered a finer stone or at least one that is capable of very keen edges. I favor Coticules but will say I have found they can range in capability from one stone to another, so when you are looking at or reading about Coti's its worth noting that its not as simple as grabbing ANY coticule and getting the results you are expecting. coticule.be does have some good information and is a good source to learn about the different types/qualities of coticules. I have found, and many others will say the same, that a hard/surgical black ark is a good compliment to a coticule honed edge and great way to kick it up a notch. PERSONALLY, I have failed to learn how to use the various types of arks so I stick to the Surgical black (really hard black ark). I would recommend getting your arks from Dan's because his stuff (and his information) tend to be quality...the LAST think on earth you want to be doing is trying to lap flat a hard ark that you got cheap from some unknown place.
Whether its a Coticule, ark or echer the stone must be dead flat.