The .357 is great for what it was made to do. If you are a big slow bullet fan than this round is not for you. However let me explain some of the things that have not been touched on. First many departments where desprate to get a large cap mag, that had the power like their old .357 mag revolvers, yet they needed to be able to have a large number of rounds and the auto's quick reload abillity. One thing that they found during the Texas DPS testing was that scores of new troopers in training got better. IE they hit the target more. One class used the SIg P220 in .45 acp and shot for score, another class used the SIG P226 .357 sig. the .357 class got better scores, than they switched guns between classes and again the .357 shooters got better scores. Texas also wanted a gun that had a better long range performance, apparently they shoot alot of coyotes at a hundred yards and with the .45 they had too much bullet drop. Then they had a shooting. two troopers engaged a gunman inside a semi truch cab. the traniee had the .357 P226, while the traning officer had the old P220 .45 acp. after the shooting they foudn that the rounds from the .45 did not make it into the cab, while the .357 rounds did and killed the gunman, yet at the same time they stayed in the body.
Because the .357 Sig round is a recent invention, we know more about bullets than back in 1910 when (about) both the 9mm, and .45 acp came out. For some reason bullets from the .357 tend to stay inside the body of person shot and not pass through most of the time. Now this does depend on bullet type, and brand. In terms of police use there are two major bullets that top the list. The first is the LE only winchester ranger 125gr "Q" load, that is used by the United States Secret Service. The second is the Speer 125gr Gold Dot. Now despite what people say the bullets are not 9mm, because of the much high velocity the specs for the .357 mag bullets were used to withstand the much higher MV. The oppisit happens with +P+, & +P 9mm loads, those rounds use the 9mm bullet but push it almost as fast as the .357 so they are much more prone to over expand and fragment. Now for my uses I do not mind the over expansion and fragmentation of the +P 9mm. However if I was partoling the local express way all day and would face bad guys behind auto glass I would be carrying my .357 sig not the 9mm.
Now in terms of the .40 S&W vs. the .357 sig it is a strange contest. In its orriginal loading the 180 grain bullet was good but not great. now if you take a 135gr .40 load and compare it to a 125 gr .357 load you will find things are very close but again the light bullets over-expand and fragment in the 135 gr .40 loads. so if you take a .40 135 load, and a 9mm 125 +P, and a .125 gr .357 sig load you will find the results are very close except the .357 bullet will go just a little deeper and hold togeather better.
As for ammo costs I have found that if you plan ahaed and order from online the .357 target rounds are not muct more than the .40 S&W loads. I just last week got a box of .357 sig, winchester target loads for 18.99 at cabelas store in Michigan. So my solution is to own a .357 and a .40 S&W barrel for each gun. I have a Glock 22, 32, Sig P226 ST and all have two barrels for them. The reason I do that is because I (and this is just a quirk of mine) like to have a handgun close to that of local police. Since michigan State police carry the P226 .40 S&W I like to have that, however if I got visit my friends in Texas, than I switch over to the .357 sig. drive west from Texas and New Mexico State Police use the Glock 31 .357 sig. Now as someone here said before in times of emergency or natural disaster good luck buying a box of 9mm at the gun shop, but almost always there is some .357. no it will not be cheap but it works very nicely. In a bug out situation being able to shoot two differat calibers is very handy.
Lastly is were I think that the round really does a good job. If I lived on a farm and wanted to protect myself and my livestock from wolves, coyotes, and bears than I would just carry my .357 loaded with the 125 grain gold dot. I have personally seen the aftermath of a 7 foot grizzly shot with this load. there was only one shot fired and the bear went down. Now this was a perfectly placed shot, and probaly involved some luck but it worked. I have also shot a number of wild hogs with this load and it does amazing things. Since I shoot an auto pistol better than a revolver I prefer to carry my sig p226 in the woods for bear protection when I hunted alot in alaska. Since anyone in the wilderness has a better chance of running into a criminal than a big bear this should also be considered.
Well I am sorry for the long post but I hope I explained to everyone what the round is useful for and what it was made for. Everyone has their own personal opinion about he best caliber for defense so some people will prefer the ballistics of the .45. I think we should fight with ourselves less and just be happy that the number of gun owners in America is growing!
Shoot strait and have a good day!