http://www.ar15.com/archive/topic.html?b=6&f=1&t=298193
soo... they're both one milled receiver. so much for all that noise.
anyway, to address the OP, I never bought into the 870 vs. 500 debate. I've had a couple years' experience with the 870, and about 1yr experience with the 500 and both are great.
my 870 is a tighter fit. it doesn't rattle. one negative is that the break-in period was obnoxiously long. I had alot of extraction issues, not with the extractor itself but with shells stuck in the chamber. it took a while but that has cleared itself up and now it runs smooth as silk.
500 has never had a problem. the only two that I have experience with are both old. one is from the 90's and the other is from '75. the lack of a loading gate is nice but not a deal-breaker, it's rattly as hell but as long as it goes bang it's not a deal-breaker. the safety is better IMO but also not a deal-breaker.
both are great IMHO.