If this is your first revolver, I would VERY STRONGLY recomend you go with the 4 inch barrel. The simple truth is that 4 inch revolvers outnumber any other barrel length because they provide the best balance between handling quality, sight radius, and ammunition efficiency.

As for accuracy, with the iron sights the longer sight radius of the 6 inch gun provides an advantage. However, if you use a reflex or optical sight, the 4 inch version will prove more accurate in aggragate over a range of ammunitions. Longer barrels do move more in response to the bullet transiting the barrel than shorter barrels, which means that the load has to be "harmonically timed" to the barrel and as barrel length increases so does the sensitivity to harmonics. IMO the biggest asset for the longer barrel is when using it for hunting, because if you're a bit marginal in terms of power and extra 100 fps. may mean the difference between a clean kill or spending time tracking a wounded animal.

Recoil and the 357 Magnum. IMO grips matter much more than a few additional ounces. The 4 inch 686 can be perfectly comfortable shooting 357 Magnums if you have the right grips mounted on it, use the grips that S&W ships with it and 15 rounds will likely be enough. The best grip for shooting the 357 Magnum I've used is the monogrip for the 500 Magnum. Fact is I like this grip so much I have 2 more coming this week, one for a new 625JM also coming and one for my 6 1/2 inch 610.

BTW, that 610 is a tank and taught me a lot about ammunition sensitivity and barrel length. It also taught me that longer barrels are also more sensitive to consistent recoil management because the bullet spends so much more time in a long barrel.