I've always used a 686, but could easily go with the GP100.
Since neither revolver is known for the frames failing, the method of manufacture of the frames is moot. To the extent that one of the revolvers is actually practically stronger than the other, the issues are design differences, not how the frames are formed.
Agreed. As far as durability, most seem to focus on the differences in metallurgy, but, really, both are plenty strong from this perspective. I believe the bigger differences are in the design elements:
1. Smiths use a single screw/plunger assembly to hold the cylinder/crane in, which is one of their weak links (even moreso with the newer spring-loaded plunger design) - with enough hard use & reloading, it can bend. Bend enough, and the cylinder won't shut, or will even fall out of the gun. I've seen both happen. Slam home push-release speedloader rounds with your weak hand into a newer S&W, and you may launch the cylinder off the front of the gun. I've seen that happen, too, to a brand new S&W. The GP100 doesn't use such a system, and it's impossible to even removed the cylinder/crane assembly until the trigger assembly's been removed.
2. Another S&W weak link is their ejector rod: It not only turns as the cylinder turns, but it's also used to lock the front of the cylinder in place. If the rod gets bent a bit, the bent rod turns against the retaining plunger under the barrel, which could affect the smoothness of the action. The GP100 ejector rod, in contrast, doesn't turn with the cylinder, nor (IIRC) is it used to lock the front of the cylinder. IIRC, the front of the cylinder is locked in place by a mechanism that links the crane to the frame, closer to where the actual force is being generated.
Other GP100 niceties include:
3. Front sight: The GP100 (at least the adjustable sight version) comes with an interchangeable front sight. Some S&Ws come with interchangeable sights, but most don't. Though they can be converted, it'd take some milling by a gunsmith (read: time & $$) to accept the Weigand base.
4. Reach to the trigger: Again, I'm going by memory, but IIRC, the reach to the trigger seemed shorter on the GP100, so one can use grips with a covered backstrap to reduce recoil, while still having a manageable trigger reach.
So, what are/were the GP100 cons?
1. I understand Ruger won't sell certain parts if they break, so you'll have to send the gun to Ruger for repair. If you send it back to Ruger, though, they'll send it back in factory configuration. No biggie if your gun is stock, but if you've spent money to tune & modify it, it's a big issue. And if piece of unobtainium does happen to break on your tuned GP100, then, you've got yourself a real headache, I suppose.
2. Historically, the fit and finish of Rugers weren't up to that of S&Ws, and the factory action's typically been rougher. The good news is they respond well to some basic smoothing & tuning. One of the smoothest revolver actions I've felt was on a bud's self-tuned GP100.