Since you want to use the Shotgun for self-defense, I'd recommend saving a bit longer and getting higher quality equipment. That said, my personal preference is the Benelli M2.
A good friend of mine has a Mossberg 930 Special purpose that also works really well and reliably.
Stoeger 2000 was mentioned - basically a Benelli System without the Benelli-induced price tag
The Winchester X3/FN platform seems to be a good Platform as well, even though I do not have any personal experience with it.
The strengths of the semi-auto are the faster rate of fire and the fact that they are easier to use when transferring shoulder-to-shoulder in ambidextrous use. It can be done with a pump, but it is a lot more difficult at the half-transfer point.