[ . . . ]
Again, it boils down to how you view your own situation. In general, though, I'd go for ruggedness, easily obtained ammo, and ease of maintenance, coupled with the most power that I'm comfortable and effective with.
What he said.
Ruggedness is crucial, and the most powerful caliber you feel comfortable with is no less important. Ease of maintenance is a distinct advantage, though for me not a deal killer. I'd quibble only over ammo availability: Just buy a few hundred rounds and store them (a hundred rounds per month, for x-months) and practice only with some of the newly-purchased ammo. Or, if you can afford the initial outlay, reload your own ammo.
Having said that, I love reading about all the technical advantages of one firearm type over another (e.g. rifle vs shotgun, shotgun vs handgun, auto vs revolver) or one caliber over another (e.g. 9mm vs .45 ACP), which accessories are musts and which are merely useful and etc. In fact, I've laid out more than a thousand dollars based on just such web-talk.
But first things first. You need to decide how your gun will be used: WRT preparation . . . is it to be carried to the grocery store? Is the gun intended to be used to defend the home from a home invasion (and if so, do you want to guard against a 1 person or 5 person invasion?). Is over-penetration an issue?
Where is your home located? Mine is situated on a knob with great visibility in all directions. I'm much less concerned with an assault by dead-enders than with a home invasion. So I have a fair number of handguns scattered around the house. And I carry a .380 at all times.
Are you most worried about TEOTWAWKI? If so, consider an AR-15 or AK-47 type rifle, with many loaded magazines right at hand.
Irrespective of your decision tree, the bottom line is this: you must practice. And then, practice some more.
Practice with a lesser caliber (.22 RF rather than, for example, a 9mm or .45 ACP) is useful. (I bought my wife a heavy S&W .22 revolver so she could refine her trigger control and perfect her sight picture. She carries a S&W J-frame, .357 Mag.)
But practice with the gun you count on to save your life or the life of your loved ones is beyond useful. It is critical.