Street wise, you really wouldn't have much difference in the two calibers as far as incapacitation. It greatly depends on the individual being shot and the individual doing the shooting. I will say that the Law Enforcement Agencies using the 357 Sig cartridge give rave reviews over it. Especially when needing tactical penetration through windsheilds, car doors, etc. In some cases the 357 Sig may have slightly more penetration / over-penetration than the 40 S&W on a human target. But I haven't read any reports of that being a problem on the street. Buy the one you want, load it with good quality defensive ammo, practice with it, and relax. Don't get too caught up in the which caliber is better debates, especially when the cartridges are so closely related and the street results show both to good performers.
There's Something Goin' On Here, and it Ain't Funny!!!
Either the 40 S&W or 357 SIG will stop an attacker equally well with proper shot placement. WITHOUT proper shot placement I'd just recommend that you keep shooting until they fall over - he won't be able to tell the difference.