I reload for both. Energy level for 9mm and .45 are about the same and an order of magnitude less than a battle rifle.
FMJ ammo really limits what a pistol can do anyway. You are a little better off with the .45 but the difference in one shot stops is only about 10%.
If you must use a FMJ round, there are some more current cartridges that would offer improvement over both the 9mm and .45.
Pretty much the FMJ is the biggest performance limiter. I hear the 40 or 10mm is better for FMJ while being smaller than a 45acp.
I’ve read that the semi-truncated cone FMJs pack quite a punch. I don’t believe they are made anymore though I do have some “target” loads in .380 ACP.