I will throw my 2 cents in the ring, and try and be objective about it.
I've owned shotguns in all the major categories (O/U, SxS, single shot, pump, and semi auto) in 12, 16, and 20 gauge. I had two Remington 870's; bought the second one as a "back up" in case the first one broke or had issues.....finally sold it, because I never needed it! The 870 is very dependable, easy to take down to clean, has a minimum of parts (as opposed to the Winchester 1200/1300 series) and is very popular so it probably wouldn't be difficult to find spare parts, if needed. I've used mine primarily as a duck gun, so it has been dunked, put away wet, dropped in the mud, dropped in the boat, dropped at the boat ramp, and it still performs. The only negative that I have to say about the 870 is that the ejector is a bit on the "flimsy" side (IMHO) and is pinned into place, so if it needs to be repaired, it needs the services of a good gunsmith. Other than that, I think they are about as close to perfection as you can get in a pump!
I have shot O/Us, and while I like them as a field gun, in the cramped quarters of a duck blind, trying to open and load a long barreled gun can be a pain. It's not so bad if you are by yourself, or standing in the reeds hiding, but in a boat or structure, it can be a bit difficult.
As for shell capability.......by law, we are restricted to 3 shots for migratory game. I used to remove the plug for pheasant/grouse hunting, but those extra couple of shells just added weight that usually wasn't needed. Not an issue at first, but after several hours of carrying, it did become one. I did a study at the trap range over the course of a year as to the percentage of birds shot with the first, second, and third shot. birds hit with the first shot were in the 95-96% range, birds hit with the 2nd shot (after missing with the first) was in the single digits; something like 7%. Those hit with the 3 shot after missing with the first and second were almost statistically insignificant (IMHO), something like . 7 or .8%. So, with that information I no longer remove the plugs from my shotguns, even where it's legal, for bird hunting.......and I rarely shoot that third shot if I miss with the first or second. (in some instances we are limited to the number of shells we can possess while afield). I generally save that third round for the bird (while duck hunting) in case the bird is crippled and trying to make a get away.....I HATE loosing cripples!
Anyway, that's just my opinion...........good luck with your search.