I have the 870 Police and I've killed more with that gun than the rest combined. It's a tank. All of that said, the Mossy is a bit cheaper and every bit as good. I also like the safety placement on the Mossy better.
As for aftermarket parts and accessories, it's almost dead even.
Make sure you get the 500 and not the other 5XX guns. They're not as good as the 500.