This is discussion comes up from time to time and is a classic example of how gun owners like to split hairs and exaggerate to "prove" that their FAVORITE is not just their favorite but is actually the "best".
The GP100 is heavier to hold off hand than the 686.
According to the Ruger and S&W websites, the 4" bbl 686 weighs a half an ounce MORE than the 4" GP100 and is 1/16 of an inch shorter in overall length. I think that weight and size are close enough so that we can PRETEND that there is no significant difference in these areas. At any rate, the GP100 is absolutely NOT heavier than the 686, nor is it appreciably larger than the 686.
Personal opinion leave a lot of wiggle room, but the group of people that believe the 686 is stronger and more durable than the GP100 is not very large. The GP100 is generally accepted to be pretty indestructible even under hard use that might be better described as abuse. The 686 is good for a lifetime (or two or three) of service if properly maintained, but it doesn't have same reputation for extreme durability and strength that the GP100 enjoys.
As far as trigger pulls, the 686 triggers tend to be better out of the box, but they are not "far superior" to the GP100 triggers nor are they ALWAYS better. I've had one 686 owner tell me that my 6" GP100 had a better trigger than his 686. On average the 686 trigger is going to be noticeably better, but not "night vs day" better in my experience.
And, if there's anything less relevant than hi-cap magazines to the argument of which revolver to buy, I can't think of it. Anyway, if we're going to get into company politics, S&W has one of the blackest histories of any U.S. gun companies. Maybe it's better, maybe it's not--as far as I can tell, the executive board is virtually unchanged from the group that signed the odious agreement with the government (when S&W was still a UK owned company) and that board, so far, has refused to publicly say that they renounce it. There's no way I'm going to give them credit for the fact that the government (under the current administration) has chosen not to enforce the agreement.
Both are fine revolvers, and both will provide a lifetime of service if properly maintained. Buy the one you like the best and shoot the best.