My first "more powerful" caliber was the 357 mag in a Colt Python. I now shoot a Colt Trooper Mark III in that caliber and love the caliber and revolver. I recommend the same path. Your choice of the 6" GP-100 is sound. Today and if I were younger, I would make exactly the same choice, but Smiths are really nice (and more expensive).
My favorite larger bore caliber is 41 magnum in a revolver. I have both single actions and double actions in this caliber. I prefer double actions overall. The 41 mag is a big step above the 357 mag in power.