I have a Security Six 6inch stainless .357 and it is one of my favorite revolvers! I also have a Speed Six 2-3/4inch in 9mm with a .357 cylinder also fitted to it, it is amazingly accurate for such a short barrel, although recoil is stout when used with full power .357 loads!
BTW what I have been told is that Ruger discontinued the Security Six line because when the 125 grain full power factory loads became popular, they supposedly caused erosion on the underside of the top strap if fired a lot. Not a problem with the 158gr loads (which shoot better in my guns anyway). I like the Security Six line MUCH better than the GP100, it is not only lighter (I DON'T like the full underlug on the barrel), but more grip styles are available. My favorite is the Pachmyer Presentation grips, which are not available for the GP100, at least not the last time I checked.