Semi-Auto Pistol vs. Revolver
The first and foremost consideration is your level of skill. Automatics offer higher magazine capacity and are faster to reload. But they are also more prone to malfunction, require more use of fine motor skills. If you are comfortable with the skill of arms and training to shoot your chosen semi-auto will be an ongoing part of your life, then I'd definitely go with the semi-auto. In the unlikely event that I'm ever in a gun fight I want to have as many rounds to fire as possible without having to worry about reloading. On the other hand, many people don't like guns, are scared of them, and may purchase one "just in case." If you are in that category and will have little if any training and experience with your handgun then I suggest that you stick with a small revolver.
Why? In a recent analysis of incidents involving the use of defensive firearms by civilians showed that the average number of rounds fired by the defender was 2, those that fired more seemed to fire until empty, and the immediate reaction of most attackers was to immediately flee or die upon being shot. So most of the time your small j-frame revolver will serve you just fine. Again I don't want to train for "most of the time" and so I choose to carry a full size automatic, but I can do that because I have spent the time to become intimately familiar with its operation and am comfortable deploying it in high stress situations.
If you are not going to put in the time on the range, stick with the revolver.
What is the Best Caliber for Personal Defense?
This is an easy one. The largest caliber you can shoot accurately, and carry comfortably. The FBI released a massive study done on shooting incidents in the US both law enforcement and civilian. Their analysis showed conclusively that the most significant predictor of lethality was shot placement, not caliber. Essentially what this means is that if you put rounds on target in vital areas of the body it doesn't matter much what size hole you poke. If you put hot lead into your attackers central nervous system or heart they are going down regardless of whether you were shooting a .380 FMJ or 10MM JHP.
Larger calibers will have more kick for sure, but this added kick also effects your ability to fire accurate follow up shots. For rounds that miss the CNS but still impact center mass obviously more damage is better, therefore a lager caliber expanding round is desirable, but not at the expense of accuracy.
Accuracy Trumps Caliber
So in the end how much gun you need is determined by how much training you are willing to do with that gun. You should carry as much gun as you have earned the ability to carry. Remember your gun is only as good as you are!
For a detailed analysis of some of these statistics check out this article at The Thinking Gunfighter