Let's see if we can make this decision even more difficult for you!
Knowing what I know and having shot and handled all of the players in question, top of the heap for me would absolutely be a USED Model 686, from a "no-dash" through the scarce dash-4, six shots... and six inch barrel.
I would pay -MORE- for a clean, used example of the above over ANY other regular production 686 or 686+ and especially over what is offered currently.
As to 6/7 shots, I prefer the nice, classic look of 6 shots and I like the extra cylinder meat and mass. And no, I do not think or hint that a 7-shot is "weak", just simply make mine the 6 shots.
I prefer the look and feel of a 6" barrel over a 4" barrel and part of that stems from the fact that I love shooting oodles of .38 Special from a 4-inch heavy barrel K-38 and I rather enjoy my six-inch 686 for .357 Magnum shooting.
I might prefer a 686 with a four-inch barrel -IF- my end goal were to shoot lots and lots of .38 and much less .357. I say this because 50rds of any ammo from my six-inch 686 is pure joy but 200 rounds or so and the massive full-lug barrel definitely gets muzzle heavy in a long session of shooting.