Whatever feels better for you. Choose the gun that you actually feel comfortable carrying and that you can hit your target with. Qualifying with a gun at a range is different from employing a gun in a self defense situation. This includes one-handed shooting, point shooting, shooting from retention, and shooting while moving.
I would probably carry the S&W .40 Shield, if I had to choose between both. More rounds than the SP101, thinner profile, and lighter weight. On the other hand, if you have issues with handling recoil (especially when shooting one-handed) the heavier revolver (potentially loaded with .38 Special +P instead of 357 Magnum) would be a better choice..
BTW: Which Ruger SP101? They come in different calibers and barrel lengths. I assumed 357 Magnum with 2.25" barrel.