I have a plain Jane blued Ruger GP100 with 4.2 inch barrel and adjustable sights that I bought used a few years back for less than $350. It has the full-length under lug as did most of the 4.2 inch GP100s.
I have polished the internals including the sear surfaces of the trigger and hammer, the hammer dog, cylinder star, cylinder latch, pawl, trigger plunger, transfer bar, hammer strut, and any frame or trigger guard assembly surfaces that these parts could bear on, as well as the channel drilled out for the trigger return spring.
I replaced the mainspring with a 10# Wolff spring, and the trigger return spring with a 10# Wolff. I shimmed the hammer, trigger, hammer dog, and trigger plunger. I also replaced the front, plain black blade sight with a Hi-Viz sight, which is very quick and easy to do. I am thinking about replacing the rear sight with a Bowen Classic Arms Rough Country adjustable sight.
I have not shot a 6" Ruger GP100 but I have handled a few and much prefer the balance of the 4.2" barrel model.
As others have said, the GP100 is tank-like in its construction and durability. Some feel it is tank-like in its appearance as well, which appeals to some but turns away others. Most people find the plain, black front sight blade (that comes stock on the Ruger GP100 with adjustable sights) hard to pick up. It can be replaced with the front sight for a Ruger Redhawk (which has a red insert) or with a variety of aftermarket front sights. Hi-Viz front sights are very popular with GP100 owners. The stock adjustable rear sight on the GP100 is not bad, but the windage and elevation screws don't hold quite as well as one would like and often require readjustment after shooting a hundred rounds or so of .357 Magnum.
I shoot my GP100 about as well as I do any revolver. The gun soaks up the recoil of full-power .357 Magnum loads very well. Mine has the original Lett grip which I like much better than the Hogue grip that now comes standard on the GP100. Ruger had to go with the Hogue when Mr. Lett died. Fortunately, a Lett clone can be purchased from Altamont if you don't like the rubber Hogue. Ruger also sells the nice, hardwood Hogue grip that they are putting on their Match Champion. I got to shoot a Match Champion against my GP100 a couple of days ago. Although the hardwood grip looked nice, my Lett grip was kinder to my hand.
I have shot a few L frame models over the years and have pawed over the Smith 686 at a gun shop or two. My opinion is only my opinion, but I prefer the trigger on my Ruger to that on any newer Smith I have dry fired or shot. I also prefer it to that of my friend's Colt Python. Of course, my trigger has had some work done (by me, none of which was very expensive) and may have been smoothed out by who knows how many dry fires and rounds shot.
The GP100 DA trigger pull might be slightly longer than that of the 686 but IMO it is smoother. I don't find that the DA pull stacks at all. In my experience, the most trigger pull resistance is at the very beginning of the trigger press, as the cylinder latch releases and the pawl engages the cylinder star and starts to rotate the cylinder. From there on back the resistance feels very consistent up to the break.
I usually try to roll the DA trigger pull smoothly but those who like to "stage" the DA trigger usually find this easy to do with a GP100. As the trigger is pressed, the point at which the cylinder latch reengages and locks the cylinder is accompanied by a perceptible and audible click. It is quite easy to stop the trigger press at that point and clean up the sight picture. From that point it is a relatively short and smooth press back to the hammer break.
I don't think one would find any appreciable difference in inherent accuracy between the 4.2" barrel and the 6" barrel model if shot from a Ransom rest. The 6 inch barrel model might be slightly more accurate for free hand shooting due to the longer sight radius, however. I think this would only be a factor if you intended to do some serious target shooting. I am no ballistics expert but from what I have read 38 Special P+ loads and .357 Magnum loads will pretty much achieve maximal muzzle velocity from a 4.2" barrel and very little, if any additional muzzle velocity will be gained from the 6" barrel.
