Several considerations come into play in choosing a defensive shotgun. For example, are you the only one who will be using it, or is it destined for use by others in the family as well? IMHO a defensive shotgun should be set up to fit the smallest (shortest) member of the family who will be using it. That has bearing on choosing the gauge (12 or 20) and physical size (stock length, forearm length) of the shotgun as well.
For example, I'm 6'2" or thereabouts, and my wife is 5'4". The house guns here (given the layout of the house there are more than one, kept hidden when we are home, and in the safe when we are away) are Remington 870s in 12 gauge. I can get away with that because my wife is an experienced shooter and 3-gun competitor and shooting the bigger bore doesn't bother her.
Stocks are cut to a 12.5" length of pull overall, including a premium recoil pad. Forearms are the longer 'field' type, rather than the short LE forearm. Barrels are 18-20" and there is nothing save a lightweight weaponlight and mount attached forward of the support hand position - no magazine extensions etc. They all have 4-shot Sidesaddles carrying Brenneke KO slugs, and three rounds of Federal LE 127-00 buck in the 4-round magazines.
These guns are older model Express guns, bought used for not a lot of $$$ (less than $200 each, mostly a good bit less as prices were less in years gone by). Getting them outfitted as described above cost about that much more per gun.
All that said, the shotgun itself is the least important part of the equation. Any reliable shotgun will do, if the shooter will do. Mindset is the most important, skillset is second, and the toolset comes in a distant third. Good training is critical in getting skills up to snuff, and proper mental preparation is essential before all the rest come into play. If you don't have it yet I'd suggest spending a few $$$ on a copy of Jeff Cooper's little book Principles of Personal Defense
- see http://www.paladin-press.com/product...fense/Handguns
. I suggest Paladin as a source because they have kept this essential little tome in print for several decades now. And shotgun guru Louis Awerbuck, who used to work for Cooper at Gunsite, penned the foreword of the current version.
As an early prep for using a shotgun defensively - which is different in many important regards from using it as a sporting arm - I'd suggest Clint Smith's offering at this point. See a preview at http://www.youtube.com/watch?v=-NSLxxDWpOI