If you have one 686, a 4" is probably the best choice. The extra 2 oz over the 3" makes a nice balance with full-sized grips and helps a little with recoil control. Recoil is actually not bad with the stock grips (Hogue rubber) and even better with hand-filling Ahrends Tactical grips. It's a little nose-heavy for round butt grips, so I settled on the round-to-square Ahrends version.

The 3" would be perfect with round butt, or even boot grips. There's nothing wrong with the sight radius or recoil control, as witnessed by the Hickok45 video, whacking the gong at 80 yards. This one is on my short list, rather than the 2.5" with the half-length extractor.

Neither are well suited for IWB. The thick cylinder tends to stick in the holster due to belt tension needed to keep a 40 oz revolver from sagging. A pancake holster is about as good as it gets for concealment - I have an Alessi CQC. The grip is not a particular problems, since it's held in close. Nevertheless, a boot grip would be better, and an inch shorter would be less likely to peek below a cover garment. The 686 is only 3/8" thicker than a SIG, and doesn't bulge any more than that pistol in an IWB.

Why two? I bought .357 change parts for my Dillon 550B, and don't think I can shoot fast enough with one revolver