Of course, we all know how current Jeff Cooper's opinions are... anyone read Cooper's Corner lately??? The guy is positively ancient in his techniques and gun/ammo preferences. He may be a great American, the father of practical shooting, etc., etc., but he is NOT who I would look to for current training and equipment needs in LE and self-defense.
Some of his advice is downright laughable, but I do understand that he writes with the bias of his out-dated training and firearm knowledge. There is often some wisdom to his rantings though, and I find him very entertaining.
Take Cooper's advice as you would your grandfather's advice on computers...with a big grain of salt. (no flame to the tech-savy elderly amongst us)
I would say that the Glock 20 handles every threat I could possibly face, with enough power and capacity to take care of things nicely. For those of you who would prefer your 1911, that's fine---I've got an old 486 that I would like to sell you. Flame away boys and girls---
I also agree with what treefrog449 said- the gun that you can carry on you, and can hit something HARD (my addition, in order to expel the mouse calibers) with everytime is the perfect defensive pistol.