Dave, I have spent many years shooting just about every caliber and type firearm, have done a lot of serious training with some of the best instructors, and have carried a firearm at all times for decades. Here are a few things I have learned.
By far the most important thing in gaining a defensive capability is training. Virtually everyone has an opinion on the subject of firearms, but all those change once they have some good training. The best training is with Randy Cain,
www.cumberlandtactics.com. There is no comparison. One class with Randy will teach you more than you can imagine.
In the interim, here are a few opinions shared by virtually everyone who knows the subject. First, for home defense, nothing can touch a shotgun. Not even close.
Outside the house, a major caliber rifle or shotgun with good sights sighted in with good slugs is far better than any handgun.
For concealed carry, a 1911 .45 is unmatched, however, anything you can shoot accurately and quickly under pressure is better than something bigger you can't shoot. If you can shoot a Glock 9mm, by all means use it. Just be ready and able to deliver head shots. Here again training is the answer.
If you talk to Randy about training, which I strongly recommend, he will invite you to a class with what you have or even loan you pistols for the class. Once you have some training, and see what other good shooters in the class can do with what equipment, then you are ready to consider what to buy. The class will save you a lot of money in the long run, as well as give you a real understanding of defensive capability.
Good luck!
Keith