Let's put some real numbers on a .45 ACP vs a 9mm. For those who don't want to follow the math here is the bottom line up front:
With service loads, a .45 ACP has about 6 percent more kinetic energy than a 9mm and about 29 percent more momentum.
Speer's .45 APC 230 Gr Gold Dot Load, velocity of 867 fps @ 25 yards: Energy = 362 ft-lbs
Speers's 9mm 124 Gr +P Gold Dot Load, velocity of 1146 fps @ 25 yards: Energy = 384 ft-lbs
I used the shortcut method for figuring kinetic energy - it's pretty accurate and very easy. Bullet weight/100 times velocity/100 times velocity/100 times 2.22 is the formula.
Momentum is simply the bullet weight times its velocity. In competition shooting, a term called power factor is used to establish the minimum momentum a load can have to be acceptable for competition. In 9mm it's 125, 000 and for the .45 ACP it's 165,000. This is about a 24 percent difference compared to the 29 percent difference for Speer's Gold Dot service loads.
So, a guy with effective body armor who is hit with a .45 ACP is going to be hit only about 25 to 30 percent harder than if hit with a 9mm. Energy is the wrong measure to use if we are talking about the ability to knock something down, momentum is right one. But, it's a surprise to many that the difference in energy between a 9mm and a .45 ACP is really small.
When it comes to terminal ballistics, this is the best online, simple and practical reference I know of - it's kept pretty current too. Well worth the read.
http://ammo.ar15.com/project/Self_Defense_Ammo_FAQ/index.htm