I have 686s with 4'', 5'' and 6'' barrels. all are full underlug. The one that is easiest to throw in the glove box, in a back-pack or under the seat is the 4''. The one that feels the best balanced and the one I prefer is the 5''. The one I shoot the best and is the most pleasant to shoot for extended periods of time is the 6''. The 6'', while many claim feels initially front heavy or awkward, is the one most folks prefer to shoot again at the range.
I agree with above.
I own a 686 4inch, have shot the 5 inch with unfluted cylinder...Nice piece!, and my bro in law has the 6 inch.
BTW, the balance of the gun will change with 6 or 7 rounds of ammo in the cylinder.