I have one of each, both 4” stainless models with rubber Hogue grips. They are pretty similar; full lug barrels, white outline rear sights, etc. The primary difference in my guns are the 6 shots of the GP and 7 of the 686+. (Ruger makes a 7-shot model, I bought the 6.)
The GP 100 has Wolff springs and a touch of action smoothing, so the DA pull is lighter than the pull on the 686+. The 686+ has a lightened trigger return spring and light polishing of the internals.
The SA pull goes to the 686+ by a little bit.
The GP 100 is slightly less accurate in my hands than the 686+ with the same loads. I think the issue with the DA pull of the GP is a pronounced stack that kicks in right after the locking bolt falls into the notch on the cylinder. I was able to pull slowly enough with my Lyman gauge to measure the initial pull stops at 8lb 13oz, then it stacks up to the final 9lb 9oz weight at the fall of the hammer.
The 686+ is just a smooth, ever heavier DA pull with no stacking at the end.
SA is pretty crisp on both. The 5oz difference is hardly noticeable.
14 shots. 158 gr SWC. 6.5 gr Unique. 10 yds DA.
I don’t have any targets shot with the GP, they are a bit blah, IMHO.
As for ruggedness, both handle some pretty stout loads, ones I will not shoot in my K frames. With similar grips, recoil is pretty much a push.
IMHO, you can’t go wrong with either one. I think the 686 is a bit more refined, but that is just to my eyes. YMMV.
Let us know what you buy and how it shoots for you when you get a chance.
Stay safe.