They're both good guns. In my opinion there's two specific use cases that might steer you towards the GP100:
1) Because the cylinder on the Ruger locks up at both the crane and the rear, in certain very nasty over-charge situations the GP100 might survive where a 686 wouldn't. Maybe. It's not a huge difference but basically the GP can retain the cylinder in place rather than dislodging under a really nasty excess handload or the like. So...if you're going to learn to handload with it, I'd go with a GP100.
2) Because the GP100 is meant to be field-stripped by the user, it has an edge in places like backwoods carry, carry around saltwater or other situations where the ability to do a total stripdown with minimal tools might be vital to keeping it running. The Ruger manual shows how to field-strip it to a level that S&W says "go see a gunsmith". Ruger includes the take-down tool you need under one of the grip panels, and even publishes a complete video on how to do it on their website for free. There's no voiding of any warrantee should you do so. The S&W is much harder to take apart and needs more and specialized tools to do so. A Ruger can be taken down wth a Swiss Army Knife or multi-pliers - you just need to get the one grip screw out.
That latter thing also means it's easy to do a home-brew spring kit and internal fluff-n-buff on the cheap that can usually take the GP's trigger feel up to the level of the S&W and often up to the standards of a good gunsmith or S&W's custom shop. See also:
http://gunner777.wordpress.com/2008/...k-of-knowledge
So all that plus the lower price? I'd get a GP100. No question. I *do* use a Ruger 357 as my daily-carry-without-fail but I'm a weirdo packing a New Vaquero
.