Everyone, up to now, have made valid points. Here is my experience. When I started with my former law enforcement agency in 1990, I was issued a new HK P7M13 9mm. Our range master had us shoot every other Tuesday, regardless of weather. I put 200-300 rounds per session through the gun, for ten years. I did not have a single malfunction of any type (other than those that were created for training purposes). In 2000, we got a new range master. He converted the entire department over to Glock 22C .40 caliber. My first magazine had a failure to eject. My second magazine had a failure to feed. After changing ammo, I would experience a problem about once every 500 rounds or so. Other officer's experiences were similar. No one in my former department ever shot anyone with either a 9mm or a .40 caliber. The only shootings that we had both involved a shotgun. One was using 00-buck from about seven feet into an 80-pound dog that had his teeth sunk into a toddler. The other was a slug into the face at about 20-yards. The guy had just shot and wounded two officers from a neighboring jurisdiction. Both shots instantly incapacitated their intended targets. There were no lawsuits in either case.

I carried both guns every day, either in uniform, or off-duty. I have always been a "one gun" kind of guy. I train with one gun, and carry one gun (not including a smaller backup gun, but that is a story for a different post). After a while, its use become a reflex. But, there is a weight and size difference. CCW requires thought to wardrobe, where you are going for the day (airports and courthouses usually require you to SECURE your firearm somewhere besides your holster), and weather.

EDC of a firearm is a pain. They are ALL heavy and bulky, some more than others. That being said, all of that inconvenience only has to pay for itself once.

Comments made by others regarding shooting lots of different guns is very important. Does the firearm "fit" your hand? Does it naturally point for you? Is it reliable? You will get as many opinions as there are shooters. Good advise (generally) and lots to consider. But we are all built differently. Choose what is reliable and comfortable TO YOU to shoot AND to carry. Equally important is a good holster that works for you.

In 2005, I moved to a new department. I was not issued a firearm, but was told to go and "buy what I wanted to carry", provided I could qualify with it. I went to range training (every Monday now) and found 20 or so officers there (we are a department of 40). No two officers carried the same firearm. So, I shot one magazine through everyone's gun. Knowing that I will be in investigations (plain clothes) for the remainder of my career was a consideration in my handgun selection. I chose an HK USP Compact .40 caliber, in a shoulder holster, with 2 extra magazines on the other side. I also carry a tactical light in my left pocket (I am right-handed). The backup gun remains the same from 1990. It was the right choice for me, due to the reasons listed above. Your mileage may (and will) vary.

After you finally decide, get quality instruction and practice like your life depends upon it. Because if you CCW, your life (or those of a loved one) may someday depend on it. Someone above mentioned two shots to the chest and one to the head. If you can reliably do this with your chosen handgun, in the rain, at night, in the wind, when it is cold, at the 20-yard line, then the difference between a 9mm and a .40 caliber is NONE. The result is the same.