It is a sickness, once started it is almost as hard to stop as smoking and booze. I've two more early Marple Firmer chisels "in the mail". This time a 1/4" and a 3/8" both are duplicates but one will replace a round necked (later year) chisel. So I do have a little excuse....."My name is Ken and I'm a.....".
I'm teaching myself to sharpen on JNATS and have been for years and as always it is/has been interesting. Sometimes I ask myself "why bother?" BTW, if I post something that is incorrect please correct me however you feel most comfortable, email, reply in the comments, it makes no never mind. I have thick skin and learned many years ago that ego is nothing but trouble. On to the chase..
What I find the biggest difference is the wire edge. On Ark stones it comes up easily and is easy to see and feel. JNATS not so much. The wire edge tends to be much smaller and because of that harder to see and feel. I'm still training myself on that one.
In truth either type stone will get you to pretty much the same place of "working sharp" in about the same time and hassle on like steels and I would bet in a blind test even the most sensitive craftsman would have a hard time beating random.
I guess then the question becomes....Why? I'm not sure I have a rationale answer. I can fall back on gobby gook, art speak, or what have you, while trying to pack sand up your ass but the bottom line is "because".
Some of the reasons I can give but do not hold water are: JNATS have a finer random scratch pattern than you are able to get with an Ark stone. True but so? Does it make a rat's ass when metal meets wood?
Ralph will like this one: I can get a better mirror finish with a JNAT vs. An Ark stone. Which is true until you bring strops on board and.....Wait for it...Some of the most prized JNATS do not leave a mirror polish but instead a slight haze.
As a artist I've always been attracted to imperfection, to seeing "the hand of man" in a body of work. Perfect is boring. Several of my most prized JNATS are about as imperfect as a stone can get and still be useable. That may be as good an answer as any.
P.S. JNAT stones are/can be a sickness as well, too damn many slippery slopes out there calling my name and not a mast to lash to.
P.P.S. I've a Bad Axe D-8 clone in the mail....Damn all my weaknesses are on full display. The only things missing are planes.