Being armed with considerable knowledge of mammalian anatomy and a little bit of knowledge about ballistics (much of the latter gleaned from this site) I, like many, think it unwise to count on any pistol shot being an immediate stop. Therefore, it seems wise to have a variety of targets in mind.
Mid- to upper-chest is a fine target, with a potential for causing a lot of damage to the heart, great vessels, and lungs. A hit there is likely to scare an assailant enough to stop the fight, but blood loss even from those organs is going to take a variable amount of time to end a fight for physiologic reasons. And there is always the possibility of it being protected by body armor.
A head shot probably has the greatest possibility of ending a fight quickly, but it is a small mobile target, plus there may even be head shots that don't end the fight immediately because of some quirk of the bullet's path or the body's reaction.
The pelvis is a reasonable target to have in mind. I wouldn't put much stock in hitting the perfect shot at the perfect angle to break a weight-bearing structure myself, but there is a significant amount of blood supply going through there to the large muscles of the legs, and to the internal organs of the area. Damage to the bladder and/or colon is going to cause some considerable pain, not just from the puncture wound itself, but also from leakage of contents.
If a fight is prolonged enough - even a few seconds - to lead a defender to a conclusion that chest shots are not ending the fight quickly enough, piling on the damage of shots to the pelvis would certainly have a chance at ending the fight through either psychological or physiologic factors.
We should never assume that the good guy is going to win every fight, though, and fight with everything we have. That might well include pelvic shots, even if the pelvis isn't the initial target.