...actually, they aren't the same gun. The 586 is made of steel which is blued, the 686 of course is made of stainless steel which requires no finish other than being either brushed or polished.
I prefer the look of blued steel and wooden stocks and dirt/fouling won't show up as easily as it will on stainless, but steel (blued or bright) will rust so it is recommended to keep them well oiled at all times.
If you have any leanings toward OCD a stainless gun will keep you up in cold sweats. You will forever be polishing the cylinder face to remove the scorching, which some will argue may prematurely wear the cylinder gap wider. I don't know about that, but i know that on a blued gun, you'll hardly notice this scorching.
In regards to old v new; I also don't buy into the "they don't make 'em like they used to." argument. S&W had lemons that made it through QC back when just as they do now. Just from a metallurgical standpoint, the steels that are in use now are superior to what types of steel that were being used 30 years ago (MIM aside for the moment) so I would argue that if you were to purchase a new 586 that is not a lemon, it will last longer (from a wear perspective) than a 586 that also was not a lemon from 30 years ago...any way you look at it, if either are reasonably cared for, either will last several generations.
My advise is to stay away from anything with an American carry designation/intent to it. Light weight scandium, MIM parts, weird alloys purported to be (fill in the blank) times stronger than steel yet (again, fill in the blank) times lighter than steel, is not something you want to entertain in a magnum firearm you intend to keep for any length of time. YMMV