bob,
There is no "best" shotgun. There are, however, lots of shotguns that are good enough, as long as the SHOOTER is good enough.
Pumps are less expensive to buy in many cases and less mechanically complex in most cases. Pumps and good clean semi's with good ammo are about equal in reliability on a statistical basis. You choose.
Any barrel length 18- 24" or so should work OK. I like 'em at 18" when I have a choice, some like longer barrels. Sights or beads, vent ribs or plain, a matter of choice. Use what you need/like.
Lots of folks make a big honkin deal over magazine capacity. Learning to hit what you shoot at, and learning appropriate tactics for home defense, is more important IMHO than having 28 rounds of slugs and buckshot on/in the gun. House guns here have normal 4-round (Remington 870) magazines, loaded with 3 rounds of buckshot, and 4-shot Sidesaddles with slugs. Magazine extensions make shotguns muzzle heavy and sluggish for a lot of people, shotguns are supposed to be well- balanced and quick pointing.
A trained and practiced shooter is the most important part of the equation, software trumps hardware in this case.
Stay safe,
lpl/nc