I aim at the center of mass of my target with every shot I take. I don't see any reason to do differently because it is a body shot, a chest shot, an abdomen shot, a groin shot, a head shot, etc - each time I shoot at the center of mass of the target area. I have to wonder - why would you ever shoot at anything but the center of mass of your intended target.
As to what you seem more likely to be asking seems to be a head versus chest shot. You shoot at the area of your opponent/assailant that is available. If everything is available and the opponent is very close in, I would suggest not bringing the gun up to head or chest levels, nor extending it for a shot, but to shoot from the hip and to hit your opponent in the body anywhere from the groin to the chest.
A bit further off and I would recommend first shooting the body. The ,mid-chest and upper belly area being the area easiest to hit while at the same time offering excellent opportunity for a shot there to incapacitate your opponent AND to allow for the widest margin of error laterally and vertically (with an opponent facing you or with his back to you) than if you shot at just the neck or head. So if you are shooting a bit off because the pucker factor has grabbed hold of you, or if your target moves just as you fire, the chances of still getting a hit are increased if aiming at the chest as opposed to the head with normally proportioned people as your target.
If this does not stop your opponent and a head shot is still an option, fire at the head next if you must, such as if it appears the target is wearing body armor or you can try something else. Another good option to consider, the something else you can try, is firing at the groin (the lower abdomen, you are not aiming at the sexual body parts) instead of the head. Firing at the groin also offers more of a chance of hitting your opponent than does a head shot. Like a chest shot, a groin shot has a lot of area above it and below it (both thighs), and to the sides laterally (the hips). Shots in this area can effectively stop an assailant.
Of course, where you shoot not only depends on what is available on your target but on things like: distance to the target, type of weapon you are firing, if you are firing from an aimed in position, if you are point shooting, if you are hip or push off shooting, if you are shooting from the draw, if you are vertically tracking and so on. Bear in mind, if you are shooting along the lines of paper punching, then when you draw or otherwise bring your weapon up to take aim or point shoot - you have plenty of time to bring it up level to be high enough to fire at your opponent’s head. This is just not going to be the case in a lot of real shooting situations in which you may find yourself. You may have to start firing at what you consider a less than ‘best’ area on your opponent to try to stop him from continuing to be a threat. If he continues to be a threat you may need to continue to fire and hope to be able to get a shot in a better area. If this does not work you may need to keep shooting until you do find an area to hit that brings about the desired result. This is one of the reasons I practice vertical tracking when possible, and vertical tracking combined with getting distance, seeking cover, using cover and so on. With vertical tracking, if close enough to fire from the hip, you can fire your first shot as soon as the gun clears the leather and is pointed at the target often resulting in a thigh or groin shot, the second shot will be higher in the abdomen, the third shot in the chest, the fourth shot in the head - of course, firing each and every shot only if the threat is still presenting itself to you. A little further back and you can point shoot while vertically tracking from groin on up to head. Further back still, you can aim in and do likewise with the vertical tracking. This way you acquire your target earlier than you would should you draw and immediately try to bring your pistol up high enough to fire at your opponent’s head. (Of course I am talking about a standing, or maybe seated, person who is your target.)
Being concerned that your first shot should be an incapacitating shot may just get you killed because you may take too much time to bring your weapon on target and fire. You may also miss because the head is an extremity and while you should shoot at center of mass of it, if your shot goes high - what will it hit? A body is wider and taller than a head in normal people. You are much more likely to score body hits, either in the chest cavity or the abdomen or the groin than you are on a head in almost all circumstances wherein all are available as a target.
I am talking a shootout type situation, not a hostage situation calling for an expert marksman with a scoped, high powered, rifle nor any other situation best suited for a specialist with special equipment and special shots.
all the best,
Glenn B