This is an interesting topic, and I keep hearing people voting for the Ruger because it is "stronger". I'm no expert in these guns, but what makes the Ruger stronger and the S&W weaker??
There is no strength difference. If anything, I think metalurgists would suggest Smith's forged frames are stronger than Ruger's cast frames... but I think this is a nearly moot point as these both these guns should outlive you and your unborn children - and history has shown this to be true.
I think the "Rugers are stronger" myth comes from something you hear all the time: "They are built like a tank." Literally speaking, they are - the lines are all about function over form and when you see a Ruger, it looks and feels like a nice, strong tool - the kind you could have bouncing around in a tool box ready to shoot at any time.
For the Smiths, they have a nicer finish and more refined design. When you pick up a shiny new Smith, it feels almost as much like a piece of art as a tool and even though it is built to take all of the abuse of the Ruger, you don't want to throw it in a tool box. You want to lay it down on a soft velvet pad. Is it any less durable or any less of a shooter? No way.
The nice thing about the GP-100 and the 686 are that they are workhorses. BOTH guns will fire full-house hot magnums all day long and ask for more. I say pick the one that you think looks the best and balances the best in your hand. Six months after you buy it, you won't remember the price difference, you will just have a gun - so make it the one you want.