The six shot 686 .357 mag is nice for nostalgia, but there is no good reason not to give yourself the small advantage of the the 7 shot cylinder since it's the same weight and size and costs only slightly more.

One inexcusable design flaw which S&W continues to perpetrate in its 6 shot cylinders is the location of the cylinder stop notch in the thinnest part of the cylinder. On a 7 shot cylinder, the notch should be located near the strongest part of the cylinder: between the chambers. This should (at least theoretically) make the cylinder stronger and more able to resist an accidental high pressure event.

The 4" barrel is much handier and easier to carry in a holster than a 6". The 5" is also an excellent choice. It gives you most of the advantages of a 6" but is almost as handy as a 4".