Had an 870 for a while, nice gun. About all you *need* is a wilson combat extra large "speed safety" (helps a lot if you are right handed). A side saddle is nice, but be careful not to over tighten the set screws or you will jam up the action (ask me how I know;) ). A big dot front sight would be very nice, but I never got around to it before selling the gun off.
The 590 is a nice gun for sure, a few extra in the tube, but IMO does not balance as well as the 870. The extra 2" on barrel "way out there" make a big difference in feel for me, but YMMV. The ghost ring/blade sight on the 590 is better than the stock bead on the 870 and there is no comparison between the 590's open elevator and the "solid" one of the 870. Finally, the tang safety on the 590 is better IMO than the cross bolt of the 870 as you kinda have to change your firing grip to manipulate it (unless you get the WC speed safety).
I personally HATE pistol grips on shotguns, but that may jut be cus I am a wuss and don't see any advantage over the traditional stock.
Because you asked...the thing I don't care for about a shotgun for HD is the OAL as you mentioned, but also the lack of capacity, and the fact that there will be overpenetration with any loads that I would use for HD (00, #3, or slugs). Besides being able to cut "the bad man" in half with the slug (which is not a bad thing by any means), I'm not sure that you gain much over a more versatile high-cap, auto-loading pistol...that can be fired from retention and has a white light set up that is much more user friendly under stress. I'm no freaking delta-SEAL-ranger-SWAT-commando, but I do have kids in the house and I am not simply barricading the bedroom door and calling 911 when I hear a bump in the night. My goal is to keep my kids safe, and then neutralize the threat and I feel a pistol is a better tool than a shotgun, for me.
Finally, a reliable auto-loading shotgun is hard to find and very expensive when you do. A pump gun is nice, but I have seen more than one person short-shuck one under stress, effectively turning it into a single shot without understanding why...which results in someone standing still (bad) with their heads down (worse) staring into the action wondering how they broke their unbreakable shotgun. People think a pump gun is the be-all end-all for HD, and while it is a capable weapon, it requires much more training to be proficient than most people think because "it looks so simple"...IMO of course. All that to say, have fun, train as much as possible, and find a local 3-gun match to test your skills and induce some stress into the picture.
