If anything, I would say the 686 is stronger than the Ruger. Less material, but better.
Both revolvers weigh the same and fit in the same holster. So there's really not much difference in the amount of the material involved. As far as the quality of the material, I don't see anyone claiming that the GP100 has issues with wear or parts breakage that would suggest that the S&W material is any better than Ruger's.
IMO, any strength differences in the Ruger come from design choices, not from differences in the amount or quality of materials involved. In other words, I wouldn't advise making choices based on differences in material or differences in the amount of materials.
Some things to consider:
The GP100/SP101/Super Redhawk revolvers I've bought have all needed some significant work to deal with rough internal finishing. The inside of the trigger return spring channel, especially, is a bumpy, gravel road the way it comes from the factory resulting in a hitchy, gritty, hard trigger pull. The hammer to frame fit is pretty loose and it benefits significantly from shims. A lot of the internals have sharp edges, so you have to be careful taking the gun apart if you want to avoid cutting yourself. I just don't hear these kinds of complaints about the S&W revolvers.
The GP100s are quite easy to take apart and work on which means that if you do want to do your own gunsmithing, it's easier than with a S&W wheelgun. This is one reason why I have bought Ruger GP100/SP101 revolvers even knowing that the triggers were going to need a lot of work.
The wiggly ejector rod of the GP100 design can be off-putting if you aren't used to it. On the other hand, it never backs out and ties up the gun as can happen with the S&W. Neither is a show stopper--just two more things to consider.
I do like the GP100 grips better than just about any grip you can get for an 686. The peg design makes it easy to get a lot of rubber between the metal and the shooter's hand and that makes for more comfortable shooting. Don't know if that's a factor in target shooting since there's really no need to go with hot loads for that.
I don't know that S&W revolvers are actually prone to going out of time if they are treated properly, but I do know that you just don't seem to hear about the Ruger revolvers going out of time. Not saying it doesn't happen, it just seems to be very rare. You do hear about it happening with the S&Ws. Had one friend whose 686 went out of time twice--S&W fixed it twice on warranty and he finally sold it. I actually bought it--I think it was something he was doing wrong, not really so much an issue with the gun. But any way you cut it, the S&W's do seem to be a bit more fragile in that respect.
All that said, if I were going to get a companion revolver for a S&W, I'd probably get another S&W. It just seems like it's going to be a nicer set with two S&Ws, one blued and one stainless. No need to learn the slightly different trigger feel of the GP, the different cylinder release, etc.
