I own a gp100 and my father owns a 686. I shoot far more than my father, so I've added a spring kit for the whopping 12 bucks and the approx 35 mins it takes to install it and a fiber front sight. I love the way the gp100 looks, I love the way it shoots, and I love the trigger in it now. ive put everything I can get my hands on through it and it runs like an absolute champ....BUT!!!! as far as after market accessories go the gp100 is VASTLY lacking in this department. I had a friend in the states make me a custom kydex holster, I had to search hi and low for comp III speed loaders and pouches only to find that safari land is basically the only people that sells speed loaders for them. I shoot IDPA/IPSC with mine, and as such I've wanted to buy an extra cylinder and have it cut for moon clips, but alas, not even ruger can send me one (and yes i know the gun and cylinder have to fit together by a gunsmith) I've also had a bit of a time finding people that can even cut a cylinder for moon clips. moon clips however are simple to find for some reason.
Now, the 686 has a better stock trigger by far, and i know after market accessories are fairly easy to find, but its god ugly. it looks like the red headed step child that was beat with an ugly stick for years on end, and as far as im concerned its uncomfortable. I've heard great things about them, but i just couldn't....pull the trigger on one..so to speak lol.
TLDR: GP100 is better unless you plan for any sort of mods. in which case id still buy the gp100...but be warned about it.
Side note-the gp100 match edition is not worth the extra money. you can buy a spring kit for 12 bucks and a fiber front sight for cheap also. unless you must have the engraving on the side dont even look at it.