i've shot both, i own a 500...
i don't like the safety position on the 870 as much as i do the safety position on the 500, though i do find the 500's safety more difficult to operate to be honest, maybe mine is just stiff
the thing that makes me choose the 500 over the 870 though is the slide release position
the 500's release is beside the left rear of the trigger guard, i can easily operate it without moving my hand from proper firing position........the 870's release is beside the left front of the trigger guard, which requires me to re-adjust my grip to operate it
i bought it used, previous owner told me he had probably 1,000 round through it since he bought it new and i've personally put probably another 500 through it, and i've never had any type of malfunction with it
i'll take a pic when i get home to show how beat up it is
if you count the gun itself, pistol grip, the shorter barrel, and the forearm, i've got around $200 in the gun, that wouldn't buy me a used 870 around here....