I've shot both Security Sixes and GP100's, and I like them both. All I can say is I bought a Security Six brand new in 1976, and it is now 1 of 4 .357's in my safe. It is also the one that I've shot the most, and I've run many hundreds, if not thousands of rounds through it. It still locks up as tight as the day that I took it out of the box. At one point about 10 years ago, the cylinder seized up on it, but that was due to an accumulation of fouling and grit from many firings and cleanings. I took the cylinder and yoke assembly apart, cleaned and relubed it, and I've had no trouble since.
As a matter of fact, all I've EVER shot out of my Six are 125 JHP's. Except for very briefly, they have all been my handloads. I load them to about 1400fps, so they haven't been "weenie" loads...I practice with what would be considered normal "combat" loads. The gun has withstood the test of time, and is still in excellent shape. Having said that, I do believe that a GP100 is a heavier, stronger gun, and I'd like to have one. But, I don't think that one needs to worry about the strength of a Security Six.