try searching this forum, you'll find lots of information.
I've owned both moss 500 and a rem 870. 500 is a better gun because both should never break on you, but the extractor on the 500 can be replaced with just a slot head screwdriver...the 870's requires a gunsmith (or someone more knowledgeable than I).
500 holds more rounds and is cheaper out of the box for a 'persuader model' comes with pistol grip if you're into that kind of thing. Holds 7 rounds + 1. Remington 870 holds about 4-5, not sure. You can get bird or slug barrels for either fairly cheap (mossberg makes barrels for 870's too, and they're good for the price).
Get the mossberg, good shotgun. Unless you want to go mall ninja and start attaching lasers and everything else to it, then you're going to want the 870 and I'd suggest a different lifestyle.