Since you already have a 4" GP100....I would keep the pre-lock S&W!!!
That being said...I've owned 3", 4", and 6" GP100's...and of them all I prefer, and still have, a 4" fixed-sight, full-lug GP100 in .357! I don't mind carrying a 1" longer revolver because the accuracy is much better, bullet performance is improved, and felt recoil is slightly less than with a 3" revolver!
The GP-100's are real nice but the Smith's seem to get more value added to them as time goes by. I like them both and lean toward the Ruger but to trade, if it was me, no way.
Keep the pre-lock 686, it is a timeless possession to have in your collection. If you want to do "something" with it, have a custom action job done by a reputable gun smith like Sand Burr Gun Ranch - Denny Reichard. You will fall in love with the 686 all over again!!!
Any pre-lock Smith is going up in price faster than new model Ruger handguns.
That may not make a difference with your decision, but I wouldn't trade it or sell it if I could keep from it.
No way, especially since the 686-3 is one of the nice early ones with no MIM parts. And from what I've heard, any stock 686 will shoot smoother than any stock GP100, not to mention that the 686 (prelock especially) is worth more.
Although I love my Rugers I would say no. I would never trade or sell a vintage gun to get something new. Vintage will usually increase in value while a new gun will often decrease in value.