Then you have to consider that 9mm ammo is usually cheaper than .380 ACP, so you can practice more shooting for less money with the 9mm. So many things to consider. =)
Pistol Specs:Diamondback DB9 9mm with 3" barrel
Diamondback DB380 380 ACP with 2.8" barrel
Hornady Critical Defense 115 grain FTX 9mm
Hornady Critical Defense 90 grain FTX 380 ACP
My testing process is pretty simple. I take one shot with each pistol at alternate ends of a SIM-TEST block that is loosely draped with 2 layers of medium weight denim. I take the shots from 8 feet away and impact velocity is measured 5 inches away from the SIM-TEST block. My SIM-TEST blocks are now closely calibrated to ballistics gel density. I shoot the blocks at the range and then bring them home and recover the bullets. The video below documents the entire process from range to recovery.
Recap of Test: