As I have mentioned several times before, I am no shotgun expert. Everything I know about defensive shotguns, I picked up in the last few months when I took a four day defensive shotgun class at Frontsight and a five day defensive shotgun class at Gunsite.
I got to try various shotguns and quickly developed preferences.
My own shotgun that I used in both courses has a factory Remington barrel with rifle sights. I personally want the ability to accurately shoot slugs, so I prefer some kind of sights other than a bead. At first I didn't like the factory rifle sights. They seemed to be harder to pick up and at one point in between the two classes the windage adjustment screw shot loose with about 50 slugs. But, after continuing to use them, I found that I actually liked them. Maybe if I would have spent more time with other sighting systems I would see the advantages, but I am pretty happy with the standard Remington rifle sights. Not that this is any kind of real definitive test, but during the man on man shoot-off at the end of the Gunsite course I took second place and ended up actually winning more "matches" than the winner because after he won, I continued shooting because there was a three way tie for second. I ended up winning all the tie breaker matches. So, against something like 19 other trained shotgunners I used factory sights to come in second. So they are not all that slow. This shoot off consisted of two shooters sitting in folding chairs. Their shotguns were loaded cruiser ready (chamber empty and safety on). At the start signal the shooters got up and engaged two poppers with a hostage in the middle, so you had to hold off with buckshot. You then did a select slug drill and shot an 8" plate at 50 yards. First 8" plate to fall with no hits on the hostage wins.
Earlier in the Gunsite class I broke my recoil pad and borrowed two guns to use while mine was being repaired. One was a Gunsite shotgun that had all the bells and whistles including ghost ring sights which I used for half of one day and I borrowed a gun from Hans Vang to use for another half day which also had all the bells and whistles on it. I found that I didn't really care for the ghost rings, but it very well might have been because I was used to my own gun. Maybe more trigger time I would have ended up liking them better ?
One problem that seemed common in our class was round front sights and ghost ring rear sights. I am talking about the front sights where the top of the front sight is kind of a cylinder, sometimes containing tritium similiar to the front sight on a Model 94 Windchester. Obviously when you are using iron sights, you zero with the point of impact at the top of the front sight. This allows you to see the target and your sight. Everyone zeroed their sights in this manner. However, under pressure, when trying to make fast slug shots, most people were shooting high. Louis Awerbuck said that the reason was that they were centering the round front sight in the round ghost ring rear sight instead of putting the tip of the front sight on the target. This sounds simple and seems like one of those things that we tell ourselves we would never do, but it happened all the time. Louis has taught thousands of shotgun classes and knows what he is talking about. So, if I had my choice with ghost ring sights, I would make sure I was getting a square front sight similar to what you would have on a handgun.
My personal shotgun is a Remignton 870 Express with an 18" Remington factory barrel. No mag tube extension. Surefire 6v dedicated foreend light. Wilderness single point sling, and a Hogue synthetic stock with 12" LOP. Six round sidesaddle.
Not really fancy, but in my limited experience seems to be the best all around set up for me.
As a homeowner or civilian, I don't really plan on carrying my shotgun a lot. But, for the purposes of these classes, these things get heavy fast. So to me, a sling is a must and I have found with both my shotgun and my AR15s, the single point sling is the way to go. YMMV. When standing on the line, I can let the weight of the gun rest on the sling giving my arms and back a rest. I would imagine the same thing would apply if I was actually carrying the shotgun in some kind of ready position. As the saying goes, the sling on a shotgun or rifle serves the same purpose as the holster does for a handgun. I wouldn't consider carrying a handgun without a holster and wouldn't consider carrying a shotgun without a sling. I found the mag extension tubes to cause a similar problem. It added weight to the gun, and the weight was in the wrong place (at the end of the barrel). I can't imagine getting into an 8 round firefight as a civilian without having any opportunity to load from the sidesaddle, so for me, I am going without the extended mag tube. If I was shooting three gun or something like that, I am sure I would want all the ammo in the gun I could get; but at present I am not shooting any of the action shotgun sports.
If you want to buy a tricked out Police Magnum so you don't have to send it away for custom work, you might want to check out what Hans Vang as to offer. He has Police Magnums with bells and whistles ready to go.
http://www.vangcomp.com/