Of all the shotguns available, I've had the most use with the Remington 870. That simple design has saved my hide on more than a few occasions.
Now, having said that. I must also point out that there are better fighting shotguns out there. If you are sold on the pump, the 870 has a great deal going for it, but I'd also take a serious look at the Benelli NOVA. I handled and shot one - impressive. I've got one on order.
There's nothing wrong with a semi-auto. They're more expensive, but certainly not more than a tricked out (and ultra-heavy) "tactical responder". In this category, the Super 90 would get top billing with the 11-87 coming in a close second.
Gabe Suarez
HALO Group
http://www.thehalogroup.com