Out of a 2" gun, my 9mm beats or equals most .357 magnum loads when fired from a 3" gun. Stands to reason the Sig would out of short barrel guns. That's why I'm not too hot on .357s in pocket guns, well, besides the fireball from hell, the recoil, and the ear splitting noise. The .357 Mag starts to perform in a 3" gun. I'm getting a tad over 500 ft lbs with my handloads (hotter than most store bought). But, with factory, I probably would still be in the 450 ft lbs range where my pocket nine gets 410 ft lbs. 4" guns are minimum to extract real performance from factory .357 magnum.
I think the SP101 with a 3 1/16" barrel is a pretty dandy little self defense revolver. It'd be my ideal for the .357 as a carry gun. My 3" gun is a Taurus M66, a little heavier, and bulkier, but I can still carry it under a loose T shirt IWB pretty easily. The SP101 would be less of a lump, though.
Like I say, I don't really think the differences in 9mm +P, .45, .357 mag or Sig, .40 S&W, or even the .38 FBI load amount to a hill of beans. They all must be well placed and it's best to put more than one into the target kill zone to get a stop. Which ever you can do that with is best for you. The .357 full house loads in even a medium frame gun is a handful for a lot of shooters. If you can't shoot it fast and accurate, you're better served with a .38 or a 9mm or .357 Sig or a .40 all of which are easier to shoot well.
JMHO of course. Hey, I LIKE the .357 magnum, though, one of my all time favorites. It was my first major caliber handgun. I shot .38s in it for a long time, though, until I finally trained myself with magnum firearms. Heck, now I can shoot .44 mag just fine, but I didn't get to that point over night.
It first took getting a place to regularly shoot and then multiple trips to the range working on it. And, I still have to admit I don't like small, light J frames in the caliber.