I bought a Service Six in the original box when I was on a mid size revolver buying path and later a Security SIX, both with 4 inch barrels. I found the Service Six well made and with HOGUE grips, a good shooter. However, it does kick a bit with .357 magnum ammo and I only use the 110 grain ammo. I also have a S&W 681 with a 4 inch and you can feel the difference. Admittedly, I had the 681 MAGNA PORTED and it also has HOGUE'S.
I think the real reason that RUGER brought out the GP 100 was the L-frame S&W revolvers. S&W was discovering that the K-frame could not handle a steady diet of .357 ammo and RUGER was picking up on that business, so they introduced the L- frame 581/586/681/686 and never looked back. This left RUGER with a superior revolver, the SIX series, but a marketing disadvantage.. S&W was really pushing the extra strength of the L-frames over both the K-frames and SIX series and it continued later on when RUGER introduced the GP-100 revolvers with some really funny or silly adds in magazines from both companies.
I shoot the K- frame S&W model 15's a lot as range guns. They are the perfect size .38 Specials with the perfect size grips for me (as long as they are wearing HOGUE'S). They are well balanced and not overly bulky. They are just not strong enough to use as a .357 magnum range gun, at least the ones they used to build.
The RUGER SIX series capitalizes on these strengths with a stronger gun without the extra bulk, but to me, they are now too light to use with either 125 grain hot loads which is what many SD shooters want a .357 magnum for or with heavy bullet hunting loads.
I can really see an advantage in a heavier gun like the S&W 686 or RUGER GP-100.
NOTE: S&W introduced a replacement for the model 19 and 66 .357 magnum revolvers based on the L-frame, but offering 7 shot capacity and only 3 or 4 ounce weight gain and they did poorly on the market, getting dropped after only a few years. I think the extra weight matters to shooters.
If I ever go back to carrying concealed a revolver, the RUGER SIX series has a lot too offer, more than a S&W model 19 or 66 does. I like the 7 shot capacity of the 686+ and the lighter weight of the non-full lugged barrel guns, but they are still a bit bulky.
Jim