Go with a pump shotgun with extended magazine tube (8-9 rounds) in either 12 or 20 guage (Only 20 I found was a Winchester 1300) Take 18" barrel, or 20". Put Ghost rings on it, and consider adding a forearm with a built in flashlight. Parkerized finish, synthetic stock. Best home protection weapon available. I potential user is female, or somewhat small, DEFINITELY go with the 20.
Wilson Combat Scattergun Technologies has a 12 guage that fits the bill. With all of the above, plus receiver mounted side saddle carrier for 6 extra rounds-@$1150. Without sidesaddle or light, @ $900-1100.
I just love these threads!
My preference is the Mossberg 500 series, because the safety and magazine catch can be operated while maintaining a proper firing grip -- something that isn't quite possible with the Remington 870. The downside to the Mossberg is that it has a few more parts to deal with when cleaning, and it uses an aluminum receiver. Semi-autos are fine, but they're expensive and heavy and the "tactical advantage" is visible more in competion than anywhere else.
Sights are a big plus on a "social" gun, but I've seen guys do amazing work with slugs with only a front bead. One thing you definately want is a dedicated weapon light -- Surefire has a dedicated forend that replaces the standard stock. Only downside to this is that the light costs more than the gun. You can also consider a "side saddle" TacStar makes a good one, but I don't really know that you're going to need a bunch of extra rounds in a home defense situation. Lots of people also find that the standard stock is just a bit too long, and a 13" "youth" stock is a better fit.
In my experience, if the stock is the proper length, and you're mounting the gun properly, the recoil from a 12 guage is easily handled by anyone weighing over 90 pounds. With an improper mount, or a stock that is too long, well, I don't care how big you are -- you'll get beat up!
Anyway, a 18" or 20" 12 guage is a fine primary. Economical when compared to most quality handguns, powerful, reliable. No reason in the world not to have a couple available.
One final thought: I've seen lots of trouble with the expensive Wilson/Scattergun front sights -- at least in the past, they were 'bonded' to the bbl, not brazed ('cause they didn't want to break the tritium bead with the heat). These fall off with some frequency. You want that front sight brazed on to the gun. Also, a small percentage (say one in 10) of Vang comp. bbls seem to 'throw a flyer' on occasion -- maybe a small burr in the bbl. You'll have a nice tight pattern, but one pellet seems to show up consistently outside of the pattern at, say 10 o'clock. Factory bbls seem to be just fine.