The 686 is much better polished & finished. My GP100 had sharp edges on the trigger and hammer that cut my fingers.
The 686 trigger is MUCH smoother and and breaks more crisply. The GP100 trigger is gritty with hitches throughout the trigger travel.
The trigger reach is a little longer on the 686, which I prefer. The GP 100 trigger travel is longer and stops just short of the frame, pinching some skin between the trigger (with sharp edges) and the frame. Ouch!
In theory, I like way the GP100 breaks down without the use of screws. In practice, mine is poorly fitted. The trigger assembly can't be removed without great difficulty.
The cylinder chambers are better polished/finished in the 686. They clean up much easier/quicker than the GP100 chambers. The GP100 chambers have tool marks.
Accuracy is probably similar, but hard to tell because my GP100 is fixed sight. That was my fault, I should have gotten the adjustable sight version.
The alleged strength advantage of the GP100, and I'm not convinced there is any, is irrelevant. The 686 is strong enough to shoot any maximum .357 load, and then some. It certainly has more than enough strength to handle anything I'm interested in shooting.
The 686, with it's forged frame, is an ounce or two lighter than the GP100.
At the time I bought mine, the 686 was about $100 more than the equivalent GP100. There is far more polish and finish in the 686 than the $100 difference.
The biggest disadvantage of the 686 are the awful RR/WO sights. I had to spend a little extra to get decent (black) sights.
I replaced the factory GP100 grips with Redhawk grips, which fit my hand a little better than the 686 Hogue grip. But I like the N frame grip of my 625 even better, which makes me think the 27/627 may be the ultimate .357.