This may not be he most popular take on the subject, but here's my .02 for what it's worth.
Regardless of caliber, gun, etc., etc., it's all about training...Training, training, training, training, and more training.
The caliber and gun you should take is the caliber and gun you're most comfortable with. There's was a good video I saw a while ago (I'll try to pull it if I can find it) where they were arguing the efficiency of .45 vs. 9mm against a bear charge. They mimicked this by trying to shoot/hit a 3 foot workout out ball that was thrown at them (this was achieved by the thrower standing behind the shooter and bouncing the ball of a surface in front of the shooter.) And wouldn't you know it, caliber didn't matter because no one could hit a moving target coming right at them.
If you've ever had the misfortune of drawing a gun in self defense, you'll know just how hard it is to overcome the adrenaline dump your body experiences. The only way to negate these effects and be effective in this kind of situation is practice and comfort with your caliber/gun of choice.
So I'd say pick whatever you feel most comfortable/familiar with and practice drawing down on a moving target a few hundred times.