I have questions from the perspective of someone that knows he is a bad shot. (My depth perception sucks, especially in any kind of situation with lighting issues.) Also, I'm rather slight in build, and would like my wife to go to the range as well. Of course budget is always a concern, but I'd rather have a 1-3 very reliable, easy to maintain options than get too cheap with something that will be difficult to maintain.
The only thing that will mitigate these things, generally, is practice. Mechanically - I'd recommend getting good optics, with depth-perception issues, you probably want to get a red-dot optic (or find a color that suits you). There are plenty of optics out there that are cheap but good quality. I suggest the SIG Romeo series. Super-durable, 50k-hr battery life, and they clock in around $100+.
Nothing will help you more than range-time.
Also - if/when you choose a pistol: pick a small round with less recoil, 9mm, or even a .380 and avoid compact sizes, get a full-size pistol to help absorb some of that recoil. That will help your aim a lot and reduce the learning curve on your perception issue. My mom is 4'11" and her favorite firearm is my Smith and Wesson .45 revolver conversion. It's a HEAVY hand-cannon, but the recoil is shockingly light. She's not shooting the heads off of flies or anything with it - but she'll drop a round center-mass easy enough.
Also, I'm unlikely to be confronted with a riot at home. If it gets that bad, we are already in open shooting war, and I'm toast anyway. More likely to see meth heads breaking and entering.
1. I have a 20 gauge and 410. I've shot 12 gauge before, even an old 8 gauge once. (I was offered a chance to shoot a 6 gauge, but given that I was 10 years old and the 8 almost knocked me down, decided to pass. ) I can handle a 12, but would rather not. I know my wife can't handle a 12. I'd think a 20 would be plenty, especially at the top of a flight of stairs. Any reason why not under my circumstance, especially considering I'm more comfortable already with the 20?
20-gauge is fine. 12-gauge is optimal. 8-gauge is for hunting T-Rexes. Stick with what you're comfortable with.
2. What's the next option--pistol or rifle? Reading all the above, the 9 mm sounds like a good option to me.
I have no current plans to apply for conceal carry, though that might change if it gets worse.
For home-defense - since you already have a shot-gun, I think a pistol (or two) is a smart move. I say two in case your wife is on board. Rifles are a good choice... but there are a LOT of caveats to that which I'll toss my two-coppers about below. 9mm, based on what you're saying above sounds solid. It's a light-round with good ballistics and any full-sized pistol will give you a lot of capacity. Of course if you get a Glock, you can get extended mags that can put 30-rds in your weapon. The most important thing for you when purchasing a pistol, far beyond anything I tell you is: Go to a range and shoot a variety of pistols in a variety of ammo-types.
9mm pistols have so much variety it will make your head spin. And this is true of .45, .40, 10mm, .357's too. Honestly, you need to get on the range and just go with what feels good in your hand. Glocks feel different than HK's which are different than SIG's etc. And keep in mind, you can modify your weapon of choice to really dial it in in terms of grips, optics, trigger - it's very common. Your local gun-range will likely have people on staff to help you figure all that out. Trust them.
Rifles - Like pistols, you need to have a clear idea about the purpose of the weapon is for. Home defense? Well what kind of home to do you live in? The environs you live in should be a real consideration. If you live in fairly close-quarters, the configuration of your rifle for optimal use might be important to you. If you want general "all-around" utility - that would be a different set of parameters. Basic AR-15's are easy enough to put-together or purchase off the rack without much fuss. But only you can decide for, in this case, "home defense" what that actually means.
Upthread I mentioned with SHARK that I'm going with two AR-15's. One is going to be a general-purpose all-around weapon. It won't have telescopic sights or anything, just a red-dot. I live in a sub-urb but I'm backed up against a wooded area with open ground, plus long wide streets. So for me, something that is close/mid-range including close-quarters seems logical. I'm also making a CQB-specific AR-pistol. Which is largely exclusively for close-quarters inside the house, but in a pinch can deal with close-range stuff if needs be.
But for range... I'm going to build a dedicated long-range rifle that uses an entirely different round and it will be specifically for hitting things a loooooong ways away. Not really designed for home-defense, I intend on starting to hunt in the near future. Filipinos and Boar are like Peanutbutter n' Jelly.