Buffalo Bore makes a .40SW - 125 gr. at 1300 FPS. It appears more manufacturers are creating 125-135 gr loads for .40 S&W to mimic the .357 sig. The smaller cross section of the .357 sig might make it go in a tiny bit further, but it also expands a tiny bit less. I would call it a wash. Both should be about the same effectiveness as a man stopper.
The advantage of the .40 is ammo choice. They go from 125 grains all the way up to 200. This gives the platform a ton of versatility. I prefer the 155-165 grain. Some people might prefer the 180-200's. I have not shot the 125's, but I intend to. I have 135 gr powrball, but am less confident in it, then I am in 155/165 gr Federal HST.
The Secret Service might have switched, but unless somebody knows what bullets they tested, the data means very little. If they tested .357 sig 125 gr against .40 S&W 125 gr, then we would have some good data points. My guess is they didn't.
One other thing, .357 sig is not .357 magnum. A magnum 125 gr bullet moving at 1650-1750 fps is something .40 S&W or .357 sig cannot touch. It is the real man stopper. If you want that kind of power in a semi-automatic, you need to move up to 10 mm.