Two separate questions
There are two questions here.
1) Do you use a firearm (whatever the caliber) with which you have practiced a lot and have proven that the magazines feed and the ammuntion feeds and the primers go off. That is, the weapon system is reliable. Secondly, you know how well you can hit with it and the 115 grain HP has a decent track record in social work.
2) Do you use the one with better effectiveness (terminal ballistics) of the bullet when it gets to the target.
Under the specified terms, the answers are different.
You specified that the particular 45 ACP is an unproven weapon and unfamilar to you with no track record of reliability or accuracy in your hands. That mandates the 9mm. The 115 grain hollowpoint is a decent round. That does not eliminate the 9mm.
On the other hand, the 45 ACP in FMJ is also a proven round. (Note that the 115 grain 9mm HP expands to about 45 caliber size often, but not all the time. The 45 ACP NEVER has been known to shrink down to 9mm size. The 45 ACP has approximateily 30 to 40% more energy than the 9mm and about 60% more momentum, twice the mass. In my mind, the 45 ACP is a far superior round. The 9mm does have an advantage in number or rounds in the magazine, ease of recoil control and is cheaper to practice with. So, if those are in your criteria, choose the 9.
When you care enough to send the very best --- .45 ACP. (Round head or dimpled, a 45 is never smaller than 45. and 230 grains is always twice the weight of 115 grains and a .45 has 60% more frontal area then the 9mm)
Just my two cents.
Lost Sheep.
P.S. by default, shoot what you know you can hit your target with. Between an unproven 45 and my Ruger Mk II, when clearing my house of strange noises in the night, I will take the one I know will go "bang" and be on target. Hard choice, but I recall reading another thread wherein a poster said "The two loudest noises you will ever hear is a gun going 'bang' when it should go 'click' and the second is a gun going 'click' when it should go 'bang'."
