I prefer a 4 - 5 inch barrel in that size frame. It feels better balanced in my hands. You really can't go wrong with a 686 of any barrel length. Go with what feels or looks best, and you'll be all smiles at the range...guaranteed!
Now I know this is a Smith forum but I'd suggest you also look at the Ruger GP100 Match Champion. It has a 4.2 " inch barrel and upgraded grips and sights out-of-the-box. There is a choice of either fixed or adjustable sights. It was my first large frame revolver and it has been great for me. It takes to Wolff springs very well!
Now I know this is a Smith forum but I'd suggest you also look at the Ruger GP100 Match Champion. It has a 4.2 " inch barrel and upgraded grips and sights out-of-the-box. There is a choice of either fixed or adjustable sights. It was my first large frame revolver and it has been great for me. It takes to Wolff springs very well!