Having owned pistols in all three calibers by different manufacturers and having been an IDPA competitor for the last 6 years, I can say that other people's opinions mean squat when it comes to buying a pistol. Go to a range that rents guns and shoot at least one in each of those calibers. With a gun that fits your hand, some people find the .45 mild, while others think the 9mm is too much. It boils down to your preferences and your tolerance for noise and recoil. The $40-$50 you'll spend renting guns to try is very well spent when you consider the depreciation on a gun you buy then sell because it wasn't appropriate for you.
Of course, if you have friends that are shooters ask them to take you to the range so you can try out their stuff.