They Are Both Nice Revolvers
I've shot both of these and carried a 4" GP100 with bobbed hammer and compact grips for about 5 years. You can't go wrong with either one.
If you love to shoot magnum rounds, and love to shoot a LOT, the Ruger will probably last longer. My dad had his 686 rebuilt at the factory 3 times, but he was a magnumholic and had to have his Ruger Security Six rebuilt twice. Also, if this is a carry gun, I would go with the Ruger because of the overall smoother profile and smaller, rounder grips.
If this will be a house and range gun, the 686 is probably better. Better sights. Plenty rugged. Nicer trigger pull.
I would choose the Ruger.