Well, a shotgun is a pretty good choice for home protection if you KNOW someone entered and you are in a barricade situation (bedroom/saferoom door closed and locked and you are behind the best cover available) but so many times it is just a "bump in the night" you here and in these situations a shotgun is more of a liability than a help as it is rather unwieldy in tight corners. This is where a handgun shines and is one reason it is a preferred tool for building clearing despite its not being the greatest stopper. I feel every person concerned with self defense needs a short gun to go with a long gun and vice versa. It is like having screwdrivers, you need both a flat and a Phillips.
Now as to type of handgun, that is a tough one as everybody has their opinion of what they are comfortable with. My opinions are thus: Try as many different guns as you can and pick what YOU like and are comfortable with. Pick not only based on how it feels in your hand but also fire it. This can change one's opinion rather quickly as some will bite you and others are rather meek in the recoil department. Also, play around with the different controls to make sure you can reach and operate them easily. Things like the slide stop, mag release, safety (if any), can all be found in different places and operate in different directions which may not be discovered upon initial observation.
As for revolver or semi-auto, it again is a personal decision. I have used and still use both and they have their good and bad points. For revolvers, they are rather tolerant of care and being dirty which is one reason they are so popular. They can also be had in a fairly powerful caliber like the 357 Magnum yet still be able to fire very light loads like the various 38 Special loads for practice and even protection. this makes them very tolerant of ammunition unlike semi-autos. Depending on the chambering and local laws, you may be able to use a revolver for hunting, if that appeals to you. Revolvers are also more accurate as a very general rule. This may not be overly important at in side the home distances but to some it can be. They are also easy to use; mostly just point and pull the trigger. Lastly, collecting your empty cases is very easy with a revolver, this is handy if you decide to reload or to sell if you don't. I have been to ranges that have bought the empty brass which usually came out to be 25% of the cost of a new box. The negatives are revolvers only hold 5-6 rounds (there are some 7 and 8 round revolvers but they are kind of few and far between and can be expensive when found) and reloading can be rather slow if needed. One can use SpeedLoaders but they can be tricky and need a fair bit of continual practice to remain proficient. Good revolvers are as expensive as many semi-autos which was not the case a few years ago. Even used, revolvers from established firms can be pricey. They can also be uncomfortable to carry if you ever decide to get a carry permit. The cylinder is not conducive to comfort for some body shapes and carry styles. The cylinder also makes the revolver a little wider than a comparable semi-auto which some find a little harder to conceal.
I still have a couple of revolvers and am unlikely to get rid of them. They have a niche in both the field and home that a semi-auto is illequiped to handle.
My favorite defensive gun is a semi-auto. They often carry more rounds than a revolver, are quicker to reload, and are chambered for many good defensive cartridges. I also like having the ability to hang a light on an accessory rail under the barrel. This makes it very convenient to identify targets and still give you the use of two hands to fire the pistol. The reliability of semi-auto pistols have increased greatly over the years though the 1911 style pistols can still be finicky. I think the models a step or two above the base models from companies like Springfield, Kimber, etc. are the best in regards to reliability and cost effectiveness. That you are very familiar with this model is another plus in its favor. The negatives of the 1911 are they can be rather finicky as to ammo type and bullet shape. This makes individual guns more likely to jam and/or limit the type of ammo you can carry. They are also not my favorite to recommend to those not "into" handguns as they can be a little more complicated than many other models. Again, this should not be a problem for you.
My personal suggestion for a semi-auto pistol is one of the other brands and models. My personal recommendations are the Springfield XD, Glock, SIG, HK, CZ, and a couple of others I can't think of off hand. Beretta and S&W make some pretty good guns but I have not found them to fit my hand well and there have been more than a few problems with them in my presence. Many have found otherwise and I have no problem with it. Just stay away from the S&W Sigma series, they have had some fairly serious problems crop up in some of these guns. I had one and it worked fine considering the price but I many have been one of the lucky ones from what I hear. The Glock and the XD have very strong followings with the XD being my personal choice. My main gripe with the Glock is the angle of the grip, it is too sharply angled for my comfort. It is also not recommended to shoot reloaded ammo in the 40 S&W versions as the unsupported chambers can result in a case rupture inside the chamber if the brass case becomes fatigued. It is also not recommended to shoot plain lead bullets out of the factory barrels as the rifling used seems to collect lead and become a partial barrel obstruction. Other than this, the Glock is a very reliable weapon and as many are used by law enforcement, there are a number of reasonably priced used guns out there. Most of the other brands have a more vertical grip which might be more comfortable for some.
As to caliber, that is a can of worms. I personally believe that anything from 38 Spl/9mm on up using a good hollow point design will do well IF the bullet is placed properly. Bigger bores can be harder to shoot well and are usually more expensive to shoot meaning less practice; a viscious continuing circle. I carry an XD 45 ACP as my duty gun and a 40 S&W in the same model for most off duty use (the 40 is a smaller frame size than the 45 ACP which makes it a bit easier to hide).
Picking a handgun is kind of like picking underwear, what I like may very well not be the one you would choose. Try as many as you can before deciding and stick to the main names. This way you can make the most informed decision and if there is a problem, someone is likely to know how to fix it.
Your shotgun is more than sufficient for stopping someone but it sounds like you want a handgun too. (who wouldn't?) I personally don't like navigating through a house with a long gun, which I've tried to see if it was practical for me. Too many corners to turn in my house.
