Well the GP-100 is more durable and designed for constant use of the heaviest .357 Magnum loads available, and will likely outlive you.
The Security Six is older, more stylistic to some, designed closer to the K frame revolvers of Smith & Wesson, and very durable in its own right, able to handle a high amount of factory load .357s without fear of wearing it out. It's also somewhat more balanced than the GP-100.
Either one is good, it comes down to personal tastes really and what you intend to use it for.
The Security Six is older, more stylistic to some, designed closer to the K frame revolvers of Smith & Wesson, and very durable in its own right, able to handle a high amount of factory load .357s without fear of wearing it out. It's also somewhat more balanced than the GP-100.
Either one is good, it comes down to personal tastes really and what you intend to use it for.