Currently, the 357 SIG has about 150-200 fps on 9mm +P loads.  Most folks assume that this means that the 357 SIG is going to have better terminal ballistics.  Testing, however, shows that this is not the case.  The 9mm already has the optimal performance pattern, and does as well as a bullet of its diameter can do.  Thus, while the 357 SIG does have more velocity at a given distance, that velocity doesn't do anything *useful*, and in fact just gives it *slightly* more penetration, but beyond 12", additional penetration isn't very useful.  Performance after barrier penetration is sometimes worse than 9mm due to the shape of the hollowpoint cavity.

The 357 SIG round has the following advantages and disadvantages compared to 9mm:

Pros:
- more velocity = flatter trajectory = less drop at long distances (from a pistol, though, this is largely irrelevant)
- the bottlenecked case potentially makes feeding more reliable.

Cons:
- reduced mag capacity
- increased flash, blast, and muzzle flip, which means more time between (likely less accurate followup) shots
- higher chamber pressures, which means more difficult extraction and higher likelyhood of kB!
- easier for dangerous bullet setback to occur, due to the small contact surface between bullet and case neck
- significantly less ammo availability
- higher ammo cost
- relatively few loads available
- reloading is complicated by the bottleneck case
- only an extremely narrow range of bullet weights is possible

The bottom line is that the 357 SIG's benefits are quite minor, while its liabilities are significant.  If you're going to accept the reduced mag capacity, you're much better off with the 40 S&W's better terminal performance.  If you're going to stay with a ~.357" bullet, you're much better off with the 9mm for a bunch of reasons.  The .357 Mag's main plus is that the extra case capacity and cartridge length lended itself well to hunting applications where more penetration is required, but the 357 SIG can't even list that as a factor.

Conclusion:
The 357 SIG round really has no reason to exist, at least not in a pistol application.  It could be argued that it would be a better carbine round than 9mm due to its higher velocity and flatter trajectory, but that's outside the scope of this discussion.

-Troy