Bird guns, barrel length or more importantly balance means a ton.
But you have single shot, pump, semi, o/u, s/s, combo. They each have they're place... strengths and weaknesses. And what makes one good may not make another good.
Most shotties will do all of that. I have an 870 great gun does everything I need it to from home defense to rabbits, deer, birds. However once I got my hands on my O/U Remington spartan, the 870 is no strictly a HD and I will never sell my O/U its not the best, in fact its more of a working mans price but I absolutely love it.
My two cents, though it may only be worth one cent. I have a mossberg 500 with a 7 and 1 capacity. It has a tactical light, a 6position collapsable stock and a pistol type grip. I feel the standard stock plus a pistol grip is the best combo when the gun is for self defense. I can really get a good firm hold on it. I feel the light is great because if it is dark I can light up where iam looking. They will be blinded but I will be able to see what I might have to shoot.
The 8 shot capacity in my opinion also makes it a good defense gun. My gun is set up specifically for defense. As far as price, mine was just under 400 before I added a few little things. I think it will do the same thing that a 1500 dollar shotgun will do. Sometimes you are paying an extra 800 dollars for a name. Iam less than a beginner in the area of shotguns for hunting but these are my preferences for a defense shotgun. Also it has a side saddle so that I have 6 extra rounds ready. I use 00 or 00 buck. This is not a perfect answer but this is what i use and why i use it. thanks to all for listening.
Dross, if you are talking shooting birds and clays, in my opinion you want a pump or semi-auto that will take different chokes. I prefer a pump as there is just something personally satisfying to me in shooting that type of gun. Both the Mossberg 500's and the Remington 870's are fun guns to own because there are so many aftermarket products and it doesn't take any gunsmithing skills to swap out most the the parts. You can buy an 870 set up to hunt birds, swap the barrel and stock, put on a magazine extension and side saddle and have yourself a riot gun with fifteen minute's work. I'm an 870 lover myself, but I won't say anything bad about the 500's as they are good guns too.
As far as bird hunting goes, walking with a buddy or two and waiting to jump a pheasant is a fine way to spend a fall morning.
If I were going for a gun for birds like you are looking for , I would go with a semi auto. Look into a Remington 1100 or 11-87. You can get these in all sorts of price ranges depending on the deal. I shoot an 1100 and have had plenty of 870's, but for wing shooting I prefer the 1100. The 11-87 will allow you to get a 3" or 3 1/2" in shell, where as an 1100 will be a 2 3/4" or 3".
if you are talking about the difference in price it would be fit and finish... most of the higher end "bird" guns have hand fitted stocks to fit and finish is awesome... most of those barrels are also fly lapped.... then on the custom doubles the soddered joint holding the two barrrels together is repeatedly melted and changed giving both tubes a very very intentional point of impact... the actions are slicked up and nothing but the best quality product goes into it.... you are also paying for someones experience doing it.... sure i can do all of those things but my quality isnt going to be a nice as a guy who has been doing it full time for 45 years....
this is where you get into the higher priced stuff....
if you are talking about semi auto guns sometimes you are paying for the name... first thing that comes to mind is the benellie m2 vs. the remmy 1100.. virtually equal but the benelli is double the price... name is a part of it, but so is importation costs, name brand, and cost of the furniture that comes on it.... the law of diminising return is most evident on these guns.
if talking about pumps.... well they are all pretty much equal and in the end you are just paying more for the features you like... i pay more for an 870 because the action is smoother, they are easier to work on, parts are cheaper, and if SHTF i feel like parts would be easier to find for an 870 than anything else.... that said, some like stoeger (benelli clone) some like mossberg.... each one comes with its own pro's and con's and the price reflects it.
hope that answered your question.
It really comes down to what you want it for. A Super Black Eagle 2 is $1500, and if you do a lot of duck and goose hunting its worth it. Its lighter, kicks less, and the action works in every kind of weather. If you want a tricked out home defence gun a 500 or 870 is perfect because your options are almost endless. Me, I go with what ever I can get a good deal on! Thats why for waterfowl I have a Rem1100 and Browning BPS10, deer, turkey, coyotes I have a Rem870 with a thumbhole stock, a singleshot H&R for deer also, doves, quail, rabbits, whatever I have a sidexside 20ga thats older than me! As far as home defence, there's a mavrick 12ga pump in the bed room. Each one does their job, but there are ones that would do better!
I have an SX3 with a nordic + 6 extension tube 11 total it holds now, I guarantee You wont be disappointed of you got one. I have 3 over unders and I still take my SX3 with me when I shoot clay, skeet, Sorting clays, Hunt. You cant find a better semi for under 1000! I would even take it over the new browning or superblack eagle. Trust me