That being said, a handgun is certainly better than no gun, and in my opinion, the best backup handgun is the one you can shoot well. Not only do you need to be able to shoot it well, but fast and under pressure. I have had several large revolvers, and my problems with them are: 1) I cannot shoot them accurately and quickly; 2) ammunition is expensive and the guns are flat out hard to shoot often.
I've shot action pistol for a few years, and it has taught me the value of putting hundreds of rounds through your gun. I would be more effective with a 9mm that I shoot all the time than a .454 that I shoot once or twice a year. A .357 is a good balance. It is a fairly hot round with good penetration, but is forgiving enough that I can easily go through 100 rounds without having to ice my wrist. I can also fire it reasonably fast and accurately on double action.