Bullet Performance

I vaguely remembered the Marshall-Sanow project, gathering data on how often various cartridges caused "one shot stops," meaning immediate incapacitation.  Dead matters less for defensive purposes than the bad guy stopping the attack immediately.  This American Rifleman article from 2012 discusses the various ways in which bullet incapacitate.  This table shows Marshall & Sanow at least preliminary data for various rounds.  The data matches what you might logically expect: .25 ACP is not very effective; .22 LR is also not so good; .38 Special and .32 ACP are better but not great; 9mm hollow points work much better, with .45 ACP HydraShok and .357 Magnum JHP at the top of the handgun calibers.  The difference between the 9mm JHPs and the top of the handgun cartridge list is not huge. 

This article points out that applying confidence intervals to Marshall & Sanow's data shows:

By using common statistical methods, I can compare the 95% confidence interval of all 9 calibers in a single table. The interesting thing about doing this is that calibers with confidence intervals that overlap must be NOT be assumed to be statistically different from one another in terms of their stopping power...Caliber 95% confidence interval.32 48.1 - 61.9%.380 62.4 - 75.6%.38 59.7 - 72.3%9mm 85.1 - 95.0%.40 89.4 - 100%.45 89.0 - 99.0%.357 96.0 - 96.0% (ie., 95.986 to 96.014)

This would indicate that 9mm and .45 are pretty darn close; I suspect that shot placement and luck will matter more than caliber.  I can see an argument for a JHP .45 over a JHP 9mm, but having twice as many rounds on target seems to be a stronger advantage.