Both are great revolvers. Smiths often get the nod for out-of-the-box fit, finish and trigger, though Rugers tune up very nicely. Rugers are renowned for their durability, but Smiths are plenty durable, too. The bigger difference, IMO, are design differences:
1. Newer S&Ws use a single screw/plunger assembly to hold the cylinder/crane in, which is one of their weak links - 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 (though to be fair, it's been on competition guns that see lots of very hard use). 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, even a little, it can bind against the retaining plunger under the barrel as the action cycles, which will obviously affect the smoothness of the action and the gun's accuracy. The GP100 ejector rod, in contrast, doesn't turn with the cylinder, nor is it used to lock the front of the cylinder. Instead, 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: 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. Ruger won't sell certain parts, so if a piece of unobtainium breaks, you'll have to send the gun to Ruger for repair. and if you send it back to Ruger, 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 can be a hassle, 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. I've shot several tuned GP100s that were as smooth as silk.