Dave...Alot depends on what you plan to do or use the gun for. Home protection, carry, or both or just target shooting. Which ever cal. you decide on, you need to practice with the gun on a regular basis. That being said, the 380 or 9mm would be cheaper to practice with. If I had only 1 handgun, I would problably choose the 9 mm as the smallest cal. that I would go with. The 40 cal is in the middle and has been gaining popularity for some years now. My suggestion to you is to go to a gun store with a shooting range and spend the money to shoot all the calibars several times and see which one you prefer. It also gives you a chance to shoot the gun you are looking at buying.
Alot of guys like the bigger calibars because they have more knockdown power in case of an emergancy. Being able to shoot the gun accurately is equally important.
And practice is not a once every two or three months deal.