Now I am wondering what the approximate difference in value, in the same condition, between the two might be.
Approximately... there would be no major difference in "value."
Value is a sticky game. If one wants to, the most respected 'book' in the industry is likely to be S.P.Fjestad's
Blue Book of Gun Values and there is either a brand new one just out, or it's set to be out momentarily.
But even that is merely one
opinion on it's value. I can tell you that the show I regularly attend is one of the finest shows in the nation and the lion's share of the Smith & Wesson revolver folks at that show don't spend a lot of time flipping through the Blue Book, although I'm sure that most of them probably own a dog-eared copy.
"Collectibility" and "value" is a living, breathing animal. Trying to pin down a number on the difference between a 586 and a 686 is futile, IMO. Similarly, throw in a 581 or 681 to confuse the issue. All else being equal, the adjustable sight revolvers likely have the edge and likely carried a slightly higher MSRP & street price back in the 80s, but once again... anyone who chases L-frame Smith & Wesson revolvers will tell you quickly how
scarce the 581 and 681 guns are. And if you are talking about -MINT- examples, it's all about what the market will bear on any given day. Trying to set a number on the "difference" is futile.
It would be like a Smith & Wesson Model 19-3 and 19-4 if everything, and I mean
everything were dead nuts equal. Most would say that in this case, the 19-3 is a little bit older, worth a little bit more. I would not argue. Now -- put a number on it.
How much more?! Uhhh. Not sure how you'd issue that number with any confidence.