In most cases, I believe the smaller the gun the bigger the bullet approach is best.
The .357 Sig is an interesting cartridge, and certainly has it's merits. This, though, is out of a full-length barrel. When you reduce the barrel length, you're essentially working your way back down to 9mm +P levels. Same bullet diameter, same average weight.
What we're dealing with in the .357 Sig vs. .40 S&W game is the old Momentum/Diameter vs. Velocity/Shock theory. In full-sized guns, both theories apply... and both rounds have very similiar stopping power.
However... the .40 doesn't CARE how long the barrel is! It's STILL going to have it's weight and diameter even if the barrel's half an inch long.
So... I'd stick with the .40, personally. That's just me, others believe otherwise. I take it a step further, in fact... the Glock I rely on is the G-30, the .45 ACP. In fact, the very slimmest gun Glock makes is the G-36, also a .45.
The most important thing of all, though, is to carry the combination of pistol/cartridge you, personally, feel most comfortable with. The entire First Shot Stop idea is mostly fantasy anyway, and you're going to shoot better and more confidently - placing your hits most accurately - with what you like. Since shot placement has always been the key to winning a gunfight, we shouldn't lose sight of that fact as we debate caliber effectiveness.
The odds are good that, whatever caliber you're carrying, if you're GOOD with it, you're likely to emerge the winner.
Flyer