I have a two and a half inch Model 19, but I've heard so many stories about forcing cones cracking from 357 use that I have relegated it to 38 special.
Since the GP100 is basically a Ruger copy of an L frame, I would make my choice between the two of them based on price. Smith & Wesson normally has a better action, but Smith and Wesson also normally costs more.
If I could afford a Smith, I would get it. I have an SP101 in 22. And a Model 18. The difference in the shootability of the guns is night and day. The Ruger is stronger, I'm absolutely sure. And it is stainless so I doubt that it will rust. But the Smith is so much nicer to shoot. And easier to hit with.