A few points:
1) .357MAG is statistically rated #1 amongst pistol cartridges for "one shot stops", based on actual shootings. .45ACP is a close second.
2) In many home defense situations, you may be asleep with no warning when the need to defend arises. Depending on where the threat is, accessing a long gun (like a 12 guage pump) may be more difficult - you can't keep it in as many convenient places as you can a pistol.
3) If you are confronted suddenly at extremely close quarters in a tight space, a long gun may be more difficult to manuever for a solution than a pistol. The long gun may also be easier for the intruder to grasp and manuever away from a solution.
4) While its true .357MAG can be a very aggressive penetrator, that's all about what bullet you shoot. You can choose frangible rounds that will reliably expend all their energy in the target and offer little danger of penetrating walls with lethal effect to bystanders.
5) As has already been pointed out, you may fire .38 Special ammo through a .357MAG revolver. This means you can practice shooting with a low power low recoil round, thus be much less likely to be intimidated by the weapon's recoil. Do
NOT fall prey to manliness here - recoil and excess muzzle blast from magnum loads can instill flinch in an inexperienced shooter no matter how much testosterone flows through his veins. You'll learn familiarity with your weapon much easier with the lower power loads, and then in a stressed defense situation adrenaline will make sure you never notice the larger blast as you shoot.
Qualifiers:
1) I do personally prefer the sawed-off 12 guage pump as my weapon of choice where appropriate. If I am awakened and alerted, but have time to arm and orient myself to the threat, that's what I am most likely to grab from amongst my many many choices. But, that's partly due to the fact I have spent many an hour practicing CQB drills with most of my appropriate inventory. If I have any doubt that manueverability in tight spaces will be an issue, I'll settle for the pistol as preferable.
2) I do personally prefer the 1911 .45ACP as my defense pistol of choice. I do so even though I also own several .357MAG and .44MAG weapons, whose stopping power I regard highly and which I am very proficient with. I choose the 1911 because of I am just a tad faster getting it out of carry and delivering multiple rounds on target than with the revolvers. Not much faster, mind you (I have on one occasion taken the money pot from a bunch of cops who were shooting bowling pins with their auto pistols while I shot my single-action Super Blackhawk .44MAG) but every hundredth of a second counts here.
Summary:
You give lack of current ownership as a factor, as well as limited experience. The revolver is the simplest repeating weapon to load and shoot. It is compact, stores and manuevers easily. The .357MAG is arguably the most reliably "man-stopper", borne out by real-world statictics. It is also easy to learn, and you can do so with low-power inexpensive ammo.
Its the best place to start your collection.
Be sure to schedule and attend a safety course, and take your family as well.
~S~ and enjoy
