I don't have much shotgun experience, but I like the top thumb safety on the 500 over the trigger guard safety on the 870. It might just be the more recent Remingtons that I've handled, but the safety seemed cheaper and harder to manipulate than the Mossbergs'.
Possibly of note - I saw today that Magpul has a new stock for the 870: http://bit.ly/xRYApS
Possibly of note - I saw today that Magpul has a new stock for the 870: http://bit.ly/xRYApS