Two ways to look at it with various detriments to each
9mm
* 15 rounds, more shots to shoot
* Weaker bullet, 3 or 4 shots before one should/may check to see if the target is 'no longer a threat'
* Better for 'multiple threats' (unrealistic though, but still true)
* Multiple hits (3 or more) will be ammo for the DA to prosecute you, you monster.
45
* Less ammo, 7+1 or so
* More potent round, most conflicts (That even require shooting) are between one bogey, close range, and rarely ever go past a few shots (IE revolvers were adaquate and kind of still are)
* Focus more on focused shots/bullet placement, less likely to spray extra shots and hurt bystanders/ricochet
In all honesty, one would be better off with a .45acp and a mindful of experience to point and shoot accurately.
*abridged, I'm late for class* =)