Bishop,
My advice (FWIW- free) is to stick with a 12 ga. and not shift to a 20 ga., UNLESS you go to a gas operated semiautomatic in the 20 ga. I say this because a primary issue with defensive shotguns is recoil. Recoil is a function of several things, but gun weight is one of the main ones- 12 gauge guns as a rule weigh more than 20 gauge guns. The operating system makes a difference too in some cases. A gas operated shotgun has the same recoil as a pumpgun (you can't cheat physics) but the gas operating system spreads that recoil over a slightly longer time frame, making it feel as if it were less.
So- for a pumpgun, get a 12 gauge. If you want a 20 gauge, get a gas gun. My recommendation for a gas gun is a Remington 1100 or 11-87 with a smoothbore 'slug' barrel. Nothing macho in that suggestion, just experience- mine and my wife's.
The reason for getting a 12 gauge is that there is low recoil ammunition in wide distribution for the 12. But there is little to none to be found for the 20 ga. IMHO the 20 ga. pump guns are a snare and delusion for the recoil sensitive, because the only (IMHO) effective defensive loads for them- slugs and buckshot- are full house magnum loads, and kick worse in the lighter 20 ga. guns than low recoil 12 ga. loads in a heavier 12 ga. gun.
My wife (60, 5'4" tall, 135 lbs) was an active 3-gun shooter in the past. When Remington came out with its spiffy little Express Youth gun in 20 gauge, she thought it would be just the ticket for a 3-gun shotgun. She was dismayed to find that it kicked much worse with buckshot and slug loads than her Mossberg 12 gauge with low recoil 12 ga. loads.
She has absolutely no trouble shooting any of the 12 gauge 'house guns' here that are the same basic size as the little Remington youth gun. Our house guns (all 870s) have 18-20" barrels with rifle sights, full length field type forearms rather than the shorter law enforcement forearms, and 12.5" length of pull stocks. The longer forearms help in that the shooter does not have to over-extend the support arm to operate the pump, and the short stock is a necessity for smaller shooters. I am 6'3" tall and wear a 37" sleeve and have no trouble shooting the short stocks, either- just keep your thumb on the knuckle of your trigger finger so you don't bust your nose in recoil. Thse guns also have premium recoil pads- KickEez in our case, but LimbSavers, Remington R3s, Pachmayer Decelerators etc. work well too.
They have no magazine extensions, heat shields or other devices that add weight out in front of the support hand. Weight on the muzzle end is felt in a disproportionate degree, keep it to a minimum in a defensive shotgun to be used by smaller or physically weaker shooters.
Our guns do have rail-mounted white lights (Streamlight rails, Streamlight M3s or LEDWave Z-5s, which weigh very little) and 4-round SideSaddles. They are loaded with 3 rounds of Hornady TAP 00 buckshot (red hull, 8-pellet full velocity) in the magazine and Brenneke KO slugs in the Sidesaddle.
Granted what works for my wife might not work for yours. My wife took up shooting in graduate school as a form of stress relief, and had her basic training at the hands of some of the best non-paid trainers I know of. Since then she has attended training from some of the best known professionals in the county as well. She is a confident and practiced shooter and I do not have to worry about her ability or willingness to use a firearm in her own defense if necessary.
It takes a while to get to that level of confidence and ability however, and it takes learning and practice. Learning to shoot is a task and a responsibility on the level of learning to drive- no one gets handed a set of keys and sent off to a NASCAR race after all. You can't rush it. But you CAN do it.
Take a look at
http://www.corneredcat.com/ . It's a web page from a THR member and your wife and you might find it helpful. I sure do...
Good luck to both of you in pursuing your ability to defend yourselves, it is a skill set of growing necessity in these times. You have done well in further securing your dwelling, but do not back off on your determination to learn what you need to know to go the full measure if necessary.
Don't be a stranger...
lpl/nc