Rugers are heavier because they are made thru the process of investment casting. S&W frames are forged steel. In terms of "strength," its a wash. A heavier gun will, in theory, absorb more recoil than a light weight gun. In pitting a 686 against a GP100, there's probably not much difference. In pitting a K-frame against a Security/Speed Six, the Ruger wins out because its heavier. Whether or not it is actually stronger is open for debate. Neither is going to blow up with SAMI spec ammunition.
The S&W has an action that is second to none in terms of tunability. If you've ever handled a PPC gun based on a K-frame, or an old cop's duty Model 19 from the day when they could get away with that stuff, then you'd probably not give the Ruger another look (I always smile when a modern day gun writer talks about how smooth the trigger is on a modern high-dollar wonder gun when I handle at the same gun and feel a trigger pull that is 15 pounds of grit). I've got an 18, a 19, and a 66 that have been worked on, and have DA triggers in the range of seven and a half to eight pounds. I've handled competetion guns that were even lighter. I've owned a Colt Python with a factory action, that even though it was buttery smooth, was not as light or as smooth as my S&Ws. A Ruger can be tuned, but not to the degree of a S&W. The Security/Speed Six triggers tend to smooth out over time and are decent for an out-of-the-box gun, but they're not in the same league as the S&W. Of all of the Ruger revolvers, the Security/Speed Six was the best of the lot in terms of factory triggers.
I worked on a Border Patrol range during the last days of the revolver being the standard issue handgun for Border Patrol Agents. On the line, there were 686s, Ruger GP100s, a significant number of Security Sixes, a few Model 66s, and occasionally a Model 19 still floating around. The GP100s were the last issue revolvers the BP purchased before going to a standard issue semi-auto. On the range, I saw the ejector rods of 686s come unscrewed once in a while. That usually happened to Agents who weren't overly intersted in guns and seldom looked at theirs between quals. The GP100s, on the other hand, had a tendency to lock up after about 30 rounds on the qualification course. They'd have to cool down before they'd work again. The qualification course at the time called for something like 72 rounds of duty 110 grain JHP Federal .357 Magnum. The only Agents who got to shoot wadcutters on the qual course were the ones with the Model 66s, most of which had been shot a lot and were getting pretty loose. For some reason, the 66s were still desirable and Agents who were issued those guns would rather have fought than "upgraded," even when their guns would spit ashtray sized chunks of lead out the gap.
Like military issue 1911s, S&Ws come with enough "slop" from the factory that they aren't affected by lead or carbon buildup unless it gets really excessive. Unlike a military issue 1911, the "slop" in a S&W revolver doesn't seem to hurt its accuracy.
If you're looking for an out-of-the-box gun at a reasonable price for home defense or plinking, the Ruger is okay. If you're looking for something more, invest the money in the S&W.