My problem with a pump shotgun for a relative newbie for home defense is developing the ability to pump it without thinking about it. If you get a pump consider shooting a flat (10 boxes) of #8s on the skeet, trap or sporting clays range until you get the hang of it. I got a Wingmaster 870 at age 16 and pumping it became second nature. When I started shooting semis and o/u again in about 2006, I found I couldn't automatically pump the old 870 when I got it out for sporting clays.
Semis are easier to manipulate, but need to be shot enough to make sure they are reliable. I've seen semis choke on various shells, and they can be as picky on ammo as some handguns. A dirty, dry or otherwise neglected semi-auto shotgun is prone to misfeed at the worst possible time.
Also keep in mind that for indoor shooting, shotguns are not "scatterguns" you can wield without aiming. The pattern at 21 feet might not be 12 inches wide, depending on load and choke. Here is one pattern I found using 00 buckshot, Mossberg 590, 18.5" barrel, choke undisclosed.
