A couple of points I'd like to make:
If you do indeed keep your weapon unloaded, I think an automatic pistol would be one of the easiest options to load quickly. It's certainly going to be faster than feeding shells into a shotgun, and probably faster than loading a revolver too, unless you're really experienced with speedloaders. Also, if you're keeping it unloaded, there's no need to leave the safety engaged. Just slap in a mag and rack the slide. And realize that an unloaded gun kept in a safe or the top shelf of your closet is not an effective defensive weapon. If your living situation dictates that you store your weapon in such a manner, you might actually be better off spending your money on hand-to-hand self-defense courses.
Regarding ammunition selection's effect on an autoloader's operation, that's going to be mainly dependent on the gun itself. Some are more tolerant of variation than others. My own preference is for plain old 230 gr. hardball. Admittedly, it does have greater penetration than some other choices, but no worse than 00 buckshot.