I have been a huge fan of the Sec-6-series guns over the last few decades. However, when I was shopping for a good used Ruger DA .357 a year or so ago, I was told that Ruger is running out of repair parts for the old Sec-6 style guns, and once they're gone, they're gone. That, combined with the wide availability of grips and holsters for newer guns that are still in production, steered me to the newer GP series guns. They are usually a bit heavier, but I think they will also last longer and give fewer problems if you are planning on feeding them a steady diet of magnum ammo.
For me, despite a warm spot in my heart the size of Texas for those old Sec-sixes, the GP-100 seemed to make more sense. If I was only an occasional shooter, who used mostly .38 ammo, and was more prone to letting sentimentality be my guide, then I'd have gone with the finest Security-six, Speed-six, or Service-six I could find and just would have tried not to think about it breaking.
This is one of the GPs I ended up with. A 3" fixed-sight that I had grit-blasted to a non-reflective finish.