A 6" barrel has a longer sight radius than a 4" barrel, which might matter for strictly target use.

Mostly, though, I think a full lug on a 6" barrel is too front-heavy. It is a throwback to the days of off-hand shooting where "hang" was a cherished stabilizing factor. I much prefer the 4" 686, which seems more balanced to me. It is similar to the "hang" of a 6" K-22 or K-38, which have no extractor shroud.

For carry, 4" would would be the max for me. 3" or less is easier to conceal, faster out of the holster, and nearly as accurate. If you like .357 and the 686, sooner or later you'll own one of each.