Used and cheap ???
500 ,,,, can't find any.
870 ,,,, yeah ,their not around either.
So their about equal .
So don't be picky , buy the one you can get your hands on .
I have both, and I think they're both excellent BUT I
give the edge to the 500 because it's lighter weight.
I carry a Maverick 20" 12 gauge (6+1 with a bird's
head grip and 14 round bandolier shoulder strap)
in my car trunk; it's a little bit "lower quality"
500 design, which is cheap ($200 new) but good.
Both are very reliable and good shotguns for sure. I personally like the Mossberg because of the locations of the safety and slide release. You don't need to move your shooting hand for the slide release or the safety. The Mossberg is also lighter.
That being said the Remington's safety is easier to reach if it has a pistol grip stock (I have one on mine) and you won't have to move your shooting hand.