Their innards are very similar. They are broken down for cleaning basically the same way.
However, the GP100 is much beefier. Frame is thicker and it has a thick underlug that runs along the bottom of the barrel. For this reason, a GP100 may not fit in a Security 6 holster. Or if it does, it will be a tight fit.
I owned a Security 6 for many years and loved it. Then I won a new GP100 in a raffle, so I gave the Security 6 to my brother.
Either revolver is well made (barring the occasional fluke that emerges from any factory) and will stand up to years of use (but not abuse).
If you can find a Security 6 in good shape, that locks up tight, you will have a great revolver. The same may be said for the GP100.
The GP100, built much stouter, will probably stand up to more full-bore Magnum loads than the Security 6. But few shooters use full-bore Magnum loads all the time. Most of the time, low-end reloads or .38 Special ammo is used.
If you plan to shoot nothing but high-pressure .357 Magnum loads, then the GP100 has the edge. It's still made by Ruger.
The Security 6 hasn't been made since 1985. I don't see many at gun shows or shops; apparently those that have them keep them.
Should you decide on a Security 6, get one with adjustable sights. The fixed sight models are okay as defense guns but they can be frustrating when you're plinking or shooting at targets because the bullet may not hit to where the sights point.
I would strongly urge you to buy one of stainless steel if you live in an area of high humidity. I live in the Utah desert, which doesn't have much moisture, but the soil is thick with salt, which promotes rust over time. For this reason, stainless steel is also good in the salt and alkali environment of the desert.
