I see the 9mm and 45ACP as being far more similar than different. There are some clear dimensional differences, but effectiveness against humans is about equal with similar type ammo.
Both are over 100 years old, both seem to fill their role well enough to have survived.
I've got defensive handguns in 9mm, 357Mag, 40, 45. No real bias but I tend to lean toward the heavy bullet weights in all of them.
