I think that reliability is a factor in why less semi's are chosen for home defense, but perhaps I should say the perception of reliability. I believe that modern semiautomatics are very reliable and many forget how easy it is to short shuck a pump. Especially if the pump isn't used that frequently and you have to grab it under the stress of a real live situation.
As for gauge, use either a 12 or a 20. Should felt recoil be an issue you may prefer a 20.
If cost is a factor, some will opt for the pump. Most pumps will work, but don't risk it on an inexpensive semi.
I have a 12 gauge 870 for home defense. If you prefer a Mossberg suit yourself. Were I buying an HD shotgun today, I might just go with a 20 gauge semi so that the wife could better manage it.
One last thing. After reading what MarkCo wrote I am adding this by edit. If you want a higher capacity magazine be very careful about magazine tube extensions and/or how the attach if you use a weapons light. I have found with my Remington that shells can hang up where the extension interfaces with the magazine tube. Yes, I know about all the little things to do so as to prevent the spring from creating a problem when attaching the extension. The newer security models have a single piece extended capacity tube. If it will fit mine I may get one
BTW, my primary self defense weapon is no longer my pump, but a 9 mm. Still, it is nice to know that the pump is there if needed.