Like others, I have had both and thought both were outstanding. I still have my GP100, shot it today and once again wondered why I ever shoot anything else. For me, it is nothing short of perfect. The two revolvers present a perfect tool for the job, but from two different engineering and design approaches. It is good to have both to choose from but difficult to choose if you have not had experience shooting both. As everyone else has said, it is a personal preference that decides but you cannot go wrong with either one. If I had money to throw away, I would probably buy another 686, but then again, I might just buy a second GP100 to have a back-up.
That's the reason I like S&W 19/66 and Ruger Security Six. Since Security Six is IMO far superior in strength and durability, for us who who like half lug barrel, along with half lug GP100, these two revolvers are only choices. Yeah, GP100 is new gun, currently in production, but despite, I prefer Security Six. It's out of production for decades, but some fine revolvers are still out there.