In a defensive rd for a handgun which is more important to consider....muzzle velocity or muzzle energy?
481 above made a correct point, seems to me.
Velocity and energy are directly related and can't be separated.
A bullet in the chamber has potential energy. Once the primer is struck that becomes kinetic energy. Kinetic energy (K) is measured in foot pounds of energy (ft.pds). This is a unit that measures the ability of a round to do work. The work of overcoming the friction of the barrel, bucking the wind, some is lost to noise, penetrating the target, expanding, etc.
As velocity increases so does the kinetic energy available to a round. This is why a .357 Magnum round is more powerful than a 38 Spl. and why a 44 Mag. is more powerful than a 44 spl.
But it's only one part of the picture. The type of bullet chosen is critical. This includes the construction of the bullet as well as it's weight. The bullet should be matched to the task.
A 230 gr. 45. acp bullet at 475 fps has about 390 ft lbs of energy available to it to work with. This is more than enough for self defense and deer or hog at reasonable distances provided the right type bullets are selected.
The weight of the heavier bullet allows momentum to work in it's favor. But here also bullet construction matters. If a jhp bullet designed for self defense against humans is used to hunt hog the bullet may expand too rapidly and decrease momentum and hinder penetration. A round needs both the proper construction and enough energy to expand and penetrate.
Penetration requires adequate energy. Energy is expended in penetration.
Energy figures tend to be more useful to the shooter with long guns than short in my opinion.