Bullet profile is a BIG factor in FMJ effectiveness. From the beginning, the 9mm had a more pointed profile while the 45 had a flatter round nose profile. Even a nonexpanding 9mm HP will provide a flatter meplat which will transfer more energy and damage more tissue than the typical FMJ.
I carry a 9mm mostly because I have hand strength issues but also because my Father survived WW2 packing a HiPower 9mm lifted from a German. He could have carried a 45 but found the 13 rounds of 9mm were preferrable. I don't know the exact circumstances but if he thought it was adequate, I won't argue with his choice.