I am no longer the great believer in the .357 magnum that I once was. In fact, much of the stuff we have all heard about the round and it's stopping power have been over-hyped a little over the years (thanks in part to folks like Evan Marshall and "Massads-A-boob" as well as various authors of gun magazine articles who didn't know any better than to repeat such rubbish). Since the CHP transitioned over to the S&W 4006 .40 caliber with 180 gr JHP bullets, their level of success in stopping perps shot up from when they were using S&W .357 magnum revolvers with Federal 125 gr JHP (info from Dr. Gary Roberts).
After taking a look at how these calibers compare, the improved performance of the .40 doesn't really surprise me. The better .40 S&W 180 gr loads seem to penetrate to between 13 and 15" in 10% ballistic gel while expanding robustly to a diameter of between .65" and .70". Loads that reliably demonstrate this performance include the Winchester Ranger, Speer Gold Dot, Remington Golden Saber and the Federal Tactical line. Again, this is 180 gr. This is excellent performance for self defense ammunition.
Now looking at the Federal 125 gr JHP .357 magnum, from a 4" barrel this load will achieve around 10" of penetration and expand to around .50". This load will also sometimes have a tendency to fragment. While fragmentation is good for wounding when it comes to rifle bullets, it is not a desirable trait in defensive handgun ammunition. Just looking at the numbers it appears the .40 caliber would be the better of the two when using this specific ammo.
But back to your original question, if I had to choose defensive ammo for the .357, I would most likely pick the Winchester 145 gr Silvertip based on available info. It seems to meet the FBI's 12" minimum penetration requirement while offering good expansion. This load seems to expand to between .60 and .65". There was one glaring case where this load failed to stop however, so I will mention that up front. In a police shooting involving a SC State Trooper, a full load of these rounds failed to kill Trooper Coates' attacker and he was fatally shot with a .22 caliber handgun. I can't recall if Coates was wounded before or after emptying his revolver into the assailant, but the assailant did survive to face trial. This may have just been one of those cases where nothing vital was hit. Unusual things such as this will sometimes occur. The law of probability will eventually pop up. But hearing stories of others who have used this load, it seems to be pretty good at putting down bad guys. This Trooper Coates failure to stop is the only example I can site in regards to this particular load, so it may have just been a fluke. BTW, in shooting various ammo in my Marlin .357 carbine, the CCI 158 gr load seemed a little underpowered in comparison to other 158 gr loads.
Another pretty decent load for 4" barrel .38/.357's is the Remington 158 gr +P lead semi wadcutter hollow points (Federal and Winchester probably offer similar performance). This is an old load that has been around for a while and probably isn't as good as say the well-developed 147 gr Winchester Ranger is in 9mm, but it does penetrate deep enough to get the job done and usually expands pretty well. Like many older bullet designs, it may suffer when penetrating heavy clothing (some bullets fail to expand when clogging with fabric) and not expand well. When these same loads are used in 2" barrel J-frames, they fail the denim test. But they do offer less recoil than full power .357 loads which may mean faster shot recovery, better accuracy and less muzzle blast.
I know much of what I have said will come as a shock to many magnum fans, as it was shocking to me when I first learned about these problems. But after looking things over carefully, I am no longer overly impressed with the .357 and find other calibers better suited to defensive use. However, with careful ammunition selection, the .357 can be a capable weapon.
-Charging Handle