Oh boy...here we go...one of the few areas where I'm a bit overly opinionated...
REMINGTON 870- w/ sidesaddle shell carrier, Vangcomp aftermarket follower, and a couple extra Wolf springs. I do not recommend the mag extensions. It's rather infrequent, but I've seem two of them bind in the last year, one was a Vangcomp, and the other was a Tacstar. When they bound, it was UGLY- but that's just the nature of a pump shotgun it seems, they rarely fail, but when they do, they fail big time. I Also find that once you put those extra rounds in the end of your barrel, you really lose quite a bit of agility in handling it. And...if you wind up ventilating some thug, it's a lot more jury friendly, especially in a place like California. If you just gotsta gotsta gotsta have a mag extension, consider the Remington factory extension, Scattergun tech, or VangComp. I consider all three to be much better made than the Choate.
Barrel-eighteen and a half inches of course is the standard, but don't be afraid to go longer if you favor more muzzle burn, a tighter choke, and a longer sighting plane. Don't forget a good gunsmith can choke an 18 inch cylinder bore barrel, if you decide you want more range out of your buckshot.
sights- If shortrange is your only concern, a standard bead will do fine, but if you want slug shooting at longer ranges consider some. another area where I'm opinionated...Buy the MMC ghost rings...they are much superior to the Scattergun Techs sites. They're expensive, but they are seriously tough, and very effective.
Stock- If you go synthetic, I like the Speedfeed stocks, that allow two rounds to be inserted on each side. Otherwise I'd stick with wood, much more jury friendly. The Remington Factory Synthetic stocks are very lightweight, nice for carrying, but not nice for shooting. Shot side by side with a wooden stock, or a heavier synthetic stock,there is a pretty clear difference.
Forearms- the Surefire forearms are pretty well done, although very expensive. Nice for target identification. Never favored it on my own though, due to that whole clean handling hang up I have. Although I will admit, there were a few times in the woods after dark when I heard very large heavy movement nearby I wish I'd had one.
One of my favorite 870s had a six shot Sidesaddle shell carrier, five shot buttcuff shell holder (secured with the shells facing left by a sling swivel, Speedfeed stock, and an eight shot mag extension (before I shied away from them. That was nineteen rounds onboard the gun. It was insanely heavy, but in the house it was kept as a barricade gun, and if I had to run with it (which I had to a few times) there was plenty of ammo aboard.
As far as the Mossbergs, I've seen more of them I disliked than I've liked. I guess I'm just a wierd SOB, cause the last two brand new ones I saw my buddy buy ejected shells out the bottom instead of chambering them, especially once the mag spring got a little tired. I've run into all sortsa people who claim to have put tens of thousands of rounds through Mossbergs without ever once experiencing that. I don't care for the gritty action of them, although they DO have a lot nicer safety to manipulate than the 870- although the VangComp extended safety does help tremendously on the Remington. I think theres an easy to percieve difference in Mossberg Vs. Remington. If you do get a Mossberg, just do yourself a favor and compare it to a Remington.
H&K- hard kicking, sharp edges, overpriced, and hard to get parts for. Yer 870 or Mossberg ever breaks, parts are a dime a dozen, yer H&K breaks and parts are harder to find and more expensive.
As far as actions, I like the Pump shotgun for the same reason I like the revolver- they both handle neglect quite well, and are a bit less finicky with tired mag springs, different ammo, and dust bunnies. The ammo is a big thing- you can shoot everything from rubber slugs, to Magnum buckshot, and every exotic round in between and not worry about the gun cycling, or the action getting clogged up with crap.
Incidentally, I LIKE the double barrels pretty well. I'm currently thinking about getting a 10 guage side by side for myself, cutting the barrel down to 18.5 and loading it with 0 buckshot...that oughta take care anything what's givin' ya' problems...hehe.. An advantage to double barrels is simple function, clean handling, and jury friendliness. I'm pretty darn confident with two shots myself, although I'd probably still carry a couple extra rounds onboard. If you do wind up with a double (which folks seldom do) I'd recommend hammerless over exposed hammers. Alot easier and safer to uncock with shaky hands when you hear something go bump in the night and then realize it was just the cat or whatever. Also quicker and safer to work the safety.
I've shot an 870 ALOT- basically half my life now. I've shot mean tempered moose in the yard, cabin raiding blackies, and most other critters that have ever walked, flew, crawled or slithered in the areas I've lived. Never shot a person yet (thank God) but it has been used to influence folks which otherwise would have been tempted to hurt lil ole me. Hence my pre-disposition towards it.
If I've insulted anyone's favorite gun during my diatribe, I do apologize- we're all lead by our own different experiences and tastes, and these are some of mine. (I hate to be one of those I'm always right, and you're wrong if you disagree with me types)