Both are excellent tools for your described purpose and for defense as well. Both the Gp100 and 686 are solid as a rock, however both can wear and tear over time. Both will be supported by the factory at no cost to you should a problem arise.
Issues to consider:
The 686, if it is newly manufactured, may have an internal lock above the cylinder release. I personally dislike the aesthetics of the internal lock, and there have been reports of lock failures. You can find older ones without the lock but you'll pay a premium.
Some 686 can be purchased with a 7 round cylinder. Most of my firearms shoot 6 or 5, but S&W has an extra round on some 686 models, which may or may not be important to you.
The Gp100 is easier to field strip and is a great design in my opinion.
Both have rabid community support.
Personally, I love my Gp100. Its had an issue here and there that Ruger fixed up for me, and I think you'll find others have had 686's with similar issues here and there. Chances are either one you get will be a great example of the quality of Ruger and S&W.
That said, go buy a Gp100
... or both!
I load mine with Speer 125 grain .357 gold dots for urban defense and 200 grain Corbon hard cast lead for woods carry.