Bullrock:
I think that you have answered your own question, since you already own an Ruger GP100 and an S&W 686! If you buy the 6" S&W 686 in question, then you will be faced with the same balance problem that you already have with the 686 that you currently own!
Since you are already pleased with your Ruger 3" GP100-then, the 4" GP100 you are also considering, would be identical to the 3-inch GP100 you now own(Except for one extra inch in barrel length!)? I own an S&W 686-3(4" barrel)and although I do like it very much, I also like the fact that the Ruger GP100 is an even stronger revolver(Whether this strength is needed or not!)! You won't go wrong with either of these revolvers! Myself, I like the added advantage of the GP100, in being much simpler to assemble/dissassemble for routine cleaning and lubrication, than is the S&W 686!
Although it is safe to say that the 686 will probably always be considered to be somewhat more valuable than the GP100(Maybe?)for a gun that you intend to shoot(Not collect!)then, I'd pick the revolver that you like the best? You are not likely to blow these two(Very strong)revolvers up-so, you might prefer obtaining a mate for your 3" Ruger GP100(The 4" GP100!)and forget getting a "Duplicate" S&W 686 to what you own now! Just my opinion(But, the decision, must be yours!). Oh, one last thing to consider, is that the Ruger "Unwritten" warranty, is even better over the long term, than is the good warranty, that S&W offers! This has been widely reported on the Ruger(And, other forums)! I have also experienced this over the years! Good luck, in choosing?
P.S.: I really like your 3" GP100 and, with the wood grips(Instead of the factory GP100 rubber grips)and, your picture is nice too!