I have a lot of shotguns (or my dad does now since I live overseas).
I have a few 870s, a few 500s and a 590(a1?), and thats just the pump guns. All of them are great choices. I like the 500 slightly more than the 870 but I think its really 6 of 1, 1/2 dozen of the other with a slight edge going to the 500 for its finish and placement of the safety. You should definatly avoid one of those pistol shotguns, they are much harder to fire accuratly (you CAN miss with a shotgun).
Before I moved, I kept a 500 or an 870 behind the door loaded with 000 buck or 00 buck. Next to the night stand was a glock or a sig in 45, 9m or 10mm. I also routinly kept an 870 in my truck behind the seat whenever I went camping or anything like that, never needed it but was nice to have.
I really like pump action shotguns and agree with everybody who says to get some training. For your sun, Id suggest a basic hunters safety class to start, then if you want, get into a defensive shooting course.