The Ruger GP-100 is a more modern and robust design. It is a stronger gun than the S&W 686 and will take a steady diet of magnums without a problem. The front sight on the GP-100 "plugs in" and can be changed in a few seconds. All Ruger plug-in sights are interchangable (Redhawk, Super Redhawk) so you can get white, yellow, blue, green, black, or black with a red insert, all under $10 each. 686 sights cost a fortune to change because they require gunsmith machining.
Out-of-the-box, the GP-100s trigger pull is harsh. A 5th grader on his lunch hour can smooth it up and install a lighter set of springs for about $10. The older 686s had very nice trigger pulls, however the new ones are about as harsh as a GP-100.
The older 686s were great guns. Each engineering change is noted by a dash number and after the 686-3, things started going down hill. First came the rubber grips, goofy extractor, then the MIM internal parts, in-frame firing pin, and now they will have shrouded barrels (much like the old Dan Wessons, though not adjustable or interchangable). Though still a decent gun, they aren't nearly as good as the -3 or prior.
Both guns are more accurate than the person holding them. I would give a slight edge to the 686. Also, the 686 comes in a 6, 7, or 8 shot where the GP-100 is a 6 shot only. The GP-100 comes in blue or stainless. The 686 is stainless and it's brother, the 586 is blue.
If I were to buy a used gun, I would go with a S&W 686-3 or older. For a new gun, Ruger's GP-100 in stainless is my choice.