The two 'kings' of the shotgun world are the Remington 870 Express and the Mossberg 500. They are Coke vs Pepsi, Ford vs Chevy. It comes down to a matter of personal prefrence.
Both are economical and extremely reliable pump shotguns available in 12 and 20 gauge. Both 12 and 20 gauge are sigificantly more powerful than any handgun, even a 44 magnum or a 50 AE Desert Eagle or a Colt 45 or any other handgun you can think of. A 12 gauge is more powerful than a 20 gauge, but it kicks more. Some people say a 12 guage is more versitile, but a 20 gauge works just as well for hunting and self defense with two possible exceptions #1 goose hunting #2 self defense in grizzly bear country.
Now, it's not quite as simple as 'get a remmy 870 or a mossy 500'
Each gun type comes in a myrid of 'configurations'
For home defense, an 18" to 22" barrel is ideal, for bird hunting a 28" barrel is ideal, for deer/bear hunting a 24" riflesighted barrel is ideal.
For home defense, an extended magazine so you can have 7 or even 8 shells in the gun is a nice benifit but not necessary. For hunting, the standard 5 shell is plenty, and in some hunting conditions you must 'plug' it to hold only 3 shells total.
For home defense, getting a flashlight and attaching it to the gun is a VERY smart idea. If you have a flashlight on your gun when hunting, the game warden will suspect you were planning on illegally hunting at night. However, most flashlight attachments can be taken off.
There are two good things to help with all this contradictory information.
#1 while not IDEAL, a standard hunting shotgun with a 28 inch barrel and a 4-5 shell capacity is still one heck of a good defensive weapon
#2 you can get one shotgun AND get 2 barrels for it if you so choose
Here is the basic mossberg 12 guage hunting shotgun
http://www.budsgunshop.com/catalog/product_info.php/cPath/37_336/products_id/46024
it is also available in synthetic (black polymer/plastic) stock because it is very weather resistant.
http://www.budsgunshop.com/catalog/product_info.php/cPath/37_336/products_id/46979
here it is in the lower kicking 20 gauge
http://www.budsgunshop.com/catalog/product_info.php/cPath/37_336/products_id/46366
here it is in synthetic stock but with a short barrel for home defense use
http://www.budsgunshop.com/catalog/product_info.php/cPath/37_336/products_id/46276
here is the same gun but in 20 guage for less kick
http://www.budsgunshop.com/catalog/product_info.php/cPath/37_336/products_id/46499
here is the same gun in a 'package' that includes one gun and two barrels, a hunting barrel or a home defense barrel
http://www.budsgunshop.com/catalog/product_info.php/cPath/37_336/products_id/46907
here is the 12 guage with a bit longer barrel and a full length magazine tube, again home defense specialty
http://www.budsgunshop.com/catalog/product_info.php/cPath/37_336/products_id/47809
here is mossberg's premiere home defense/military/police shotgun including a special 'ghost ring' sighting system
http://www.budsgunshop.com/catalog/product_info.php/cPath/37_336/products_id/50663
******************NOTE*************
whatever you do, do NOT get a Pistol-Grip-Only shotgun, with no stock you cannot aim it and control it. Sure, you can hip fire if you want to make a youtube video of youself looking cool (or foolish to anyone with knowledge) but it is NOT very accurate or controllable, at least not when your life or the lives of your kids may be on the line
PGO shotgun
http://www.budsgunshop.com/catalog/product_info.php/cPath/37_336/products_id/47090