Between those two, GP all the way
In my opinion, the .357 cartridge was intended for a 6" barrel. Going to 4" cuts a substantial amount of velocity and increases the muzzle blast and flash substantially. A bit of a consideration when considering shooting indoors.
12 Gauge is generally best for home defense. Easier to hit with than any handgun and even in light loads, 4 times the energy of a .357 magnum.
Having said that, I love my Rugers. My house gun is a Security Six 6" (and I have a choice of
Security Six in 2.75", 4" and 6"
GP100 in 4" and 6"
Colt Trooper in 8"
Redhawk in 44 Magnum 5.5"
SP101 in .357 Magnum 3"
Between the SP and the GP, the SP makes virtually no sense as a house gun because it's light weight and small grips make it difficult to shoot well. The GP is a LOT easier to hang onto. Plus, the GP's double action trigger pull is far superior (the physically larger size just makes it so)
Good luck
Lost Sheep
P.S. You may have already given a lot of thought to the other factors of home defense, but in case you haven't: There is a lot more to home defense than a firearm. A good perimeter (or two) is essential. A plan of action that covers all scenarios, including fire, natural disaster, daytime or nighttime burglary or home invasion and the whole range of things that can happen. Flashlight, cell phone, gun, video camera, outside alarms, window alarms, proper locks, who you have on speed dial, having, joining or starting a neighborhood watch, all are factors in effective home defense.