The topic of which shotgun is better often comes up on most internet boards concerning survival. I am a Mossberg man myself, having grown up shooting the model 500 and later the 590 I have had great success with these shotguns. One of the better aspects of the 500 over the 870 IMO is the location of the safety. Having the safety located on the tang allows very easy access for me and I am able to engage the button very quickly with my thumb while bringing the gun to my shoulder.
I found this site that has a nice write up on the 500 vs the 870 if anyone is interested in reading it.
http://www.tacticalshotgun.ca/content_nonsub/shotguns/compare_870_590.html