When it comes to picking a round to carry for defensive or duty use, I have a fairly specific set of criteria I am looking for. The number one item on the list is reliability. In a semi auto that entails reliability in functioning the firearm, and ignition reliability. With a revolver, since the functioning of the firearm isn’t dependent on the ammunition being used, that part is usually pretty simple. You just have to make sure bullets don’t jump crimp or have primers back out. The reliable ignition is still a significant consideration though. Most of the time, we keep our ammunition packed away in a closet or some other storage space in nice little boxes. That isn’t true of the ammunition we carry we on our person in our defensive firearms though. That ammunition is exposed to all the same elements that we are. Changes from hot to cold and back again, rain, humidity, sweat, all sorts of things. If our ammunition becomes compromised because the primer and/or powder charge were exposed to moisture, we are going to have problems when we are already having a really big problem. Problems on top of problems are no bueno. We can’t change ammunition being exposed to environmental elements, but we can look for design characteristics to mitigate the risk, namely, sealed primers and case mouths. This protects both the primer, and the powder charge from moisture, firearm lubricant, etc. It is cheap insurance.
After reliability we have to worry about accuracy. The round used can have a huge effect on the accuracy of the firearm. I had a Glock 23 that hated anything 180gr, but 
would shoot faster 165gr and especially 155gr like a laser. Shooting revolvers, I have also found some rounds to be more accurate than others, with a revolver this is usually the result of a more consistently loaded round due to associated higher QC standards. The more consistently manufactured a round is from one to the next, the more likely it is to be more accurate. This is what makes match grade ammunition, match grade. The rounds from one to the next are very consistent, and therefore predictable. So all that to say, buy quality ammo. Cheap ammo is usually cheap for a reason.
The next part of this is shootability. I am referring to the ability to shoot the gun/ammo combo well. Shootability is kind of a subjective standard. It will depend on what gun is being used, and the ability of the person shooting the gun. I probably am not going to run a hot .357 magnum load in a J-frame revolver. Neither am I likely to run a match lead full wad cutter in a 4″ revolver because I get away with more juice behind the bullet in a larger, heavier gun. For a full size K-frame I think the break over point is going to be a mild .357 load. Any more recoil impulse than that and I am giving up too much, the gun will get out of control, and my shooting performance will degrade. In a smaller revolver, like a J-frame or LCR, I may run a light full wadcutter, or lighter weight .38 Special like Hornady’s 110gr Critical Defense.
Outside of recoil, we also have to consider muzzle flash for use in low light environments. Usually, this issue is overblown. The idea is that if the muzzle flash off a specific round is too big and bright, it can effect our ability to see. I have shot a lot of ammo through guns at night, some with a decent amount of muzzle flash. The only time I really noticed it was when I shot a cylinder full of Barnes Vortx 140gr .357 Magnum ammo out of a 2″ barreled revolver. So while not a common issue, and definitely not something to geek out about, it is something to keep in mind. The fix is pretty easy, run a load with a flash retardant powder. Most modern defensive loads will have it. Some of the older and/or budget loads may not.
There is also one more category to consider, at least with a revolver, and that is reloadability. The profile of a round can greatly impact how easy it is to reload the gun. For example, a J-frame loaded with full wadcutter ammo will be a pleasure to shoot, but you probably don’t want your reload to be the same ammo. The lack of any bullet profile makes it really hard to load in a hurry. While not as bad, I found the same to be true of the venerable 158gr LSWCHP. A round like the Hornady Critical Defense with its pointed nose will work best. Other JHP designs that don’e have a huge meplat would probably work okay too.
Oh yeah, and don’t forget wound ballistics. Having a round that performs as it is supposed to is also important, and hard to find when you get into the 2″ or so barrel lengths. Generally speaking, .357 Magnum will be more bark than bight from barrels less than 4″. Lighter weight rounds may over expand and not penetrate to an appropriate depth. Heavy rounds may not reliably expand. You can really geek out on ammo selection, try not to. Find something that is known to work reasonable well, and roll with it. Believe it or not, the full wadcutter is often recommended for snub guns because of the low recoil, and because it is not dependent on expansion.
The final piece of this puzzle is availability. I like to find a round that is readily available locally if possible. If I can’t find something locally, I want it to be something I can find easily online and that there seems to be a good, steady supply of. Carry ammunition has to be replaced over time, and having something that is hard to find, or that is hit or miss on the supply side, can be a pain to deal with.
To rehash really quickly, reliability is king, accuracy is important, the ability to shoot it without unduly compromising performance is preferable, everything else is nice to have. At the end of the day, how well we are able to shoot the gun under duress is more important than anything else, so go practice.
Priority: Performance 