I would get the 4" GP100. I, personally, like the looks and function of the GP100 over a 686. The GP100's beefy frame, that has no side plate or lock hole, looks cleaner to me. The way the barrel on the GP100 has that slick angle on the end that makes it look mean just works. I like the quick change front sight on the GP and I love the push button cylinder release that Ruger uses.
And I don't buy the "S&W trigger will be better" argument. I've tried them side by side and I didn't think the Smith had anything over the Ruger.. Maybe in the past this was true, but compare a recent manufactured model from both companies today and make up your own mind.
That all said, The 686 is a nice gun too.... and in a world were the GP100 didn't exist , the S&W 686 (or more likely the 586) would be my choice over the next medium framed .357 revolver.