Lots of good comments in above posts.......
In my training and experience it is best to stay with the same weapon and carry location all the time so you don't become confused under stress trying to figure out today's weapon location or function. As far as extra ammo, that is more about piece of mind vs. reality......The average shooting only involves 2 or 3 rounds and many of them only one shot is fired. So there are out of the ordinary examples where more rounds are fired but that is why they make the news.
I encourage people to overcome the urge to carry large amounts of ammo by developing their shooting accuracy and confidence in what ever caliber they choose to bet their life on such as a hit with a .9mm beats a miss with a .44mag. That being said use the largest caliber you can effectively hit with and it has a proven man stopping ability. My personal choice is a quality .45acp with hollow-point ammo that has been shot many times through the carry pistol so I have faith in it's function.
For carry location I advocate for an inside the waistband strong side carry since you can reach it in almost any position,standing,sitting,kneeling etc. You also have more control over the weapon and can keep someone from grabbing it from you. Your natural targeting motion will have you raising the weapon up and down until you fire so you will be on target quicker and stay on target longer than if you carry cross draw where you are moving the weapon horizontally and are moving across the target. Most people aren't as wide as they are tall so you have to work harder to stop the weapon on target and get a hit.
I agree that the best defense is being observant and trying to prevent getting yourself into bad situations or using your bike/ car to leave the area before things happen.