Well, I'll get the ball rolling, though I don't know much about Rugers.
I do know that most of what I've read suggests that the GP100 was introduced by B.R. NOT because of any deficiency in the S6, but as a counterpoint to the new, more robust S&W L-frame (which WAS introduced to address a specific weakness in its K-frame predecessor).
The Six-series is also widely rumored (including in an interview with B.R.) never to have made the company much money, though they sold better'n a million of them. The GP is supposed to have at least corrected this economic deficiency, from Ruger's POV.
Assuming both guns are 'just as good', I spose it depends on what you want it for. I got a S6 because I want to carry it a lot, and it's clearly a bit friendlier to carry. I've heard folks gripe about the grip of the Six series in comparison to the GP100, and I spose I can see that--though there are aftermarket grips out there.
If your purpose in a .357 is to see how many mega-loads you can shoot before you develop some kind of repetitive stress disorder, the GP100 would be the better bet, I'd think. I chose the S6, so I guess my choice is already clear. Let's see what someone who knows both systems better will say.