I understand your logic about a 6" M19, however you already have a 4' that has a special attachment for you, a purchased 6' would not. Since you enjoy having a wide variety of S&W revolvers, it might make more sense to buy something you don't already have. The M27 was the top of the line revolver for S&W, having the high polish blue (M28 does not) and the unique checkered top strap. The 27-2 was that last engineering change that came with the desirable pinned barrels and recessed cylinders (P&R). In addition to a 6", you might consider the 5" M27 as they are considered by many to be the best .357 revolver ever made (though Python fans may disagree).
ps-don't like the full underlug look myself, though the 586 seems to pull it off much better than the 686. And will never understand the styling direction they went with on the 627...