get the 590 if you have the cash. it has a bayonet lug and a thicker barrel than the 500. the 590 passes the law enforcement and military test the 870 did not.
with the 590 you can extend the tube too. the 500 you cannot unless its a 590 barrel and mag tube on the m500. some come stock like that.
500/590 is easier to take apart and clean too. plus the safety is in the center so leftys and rightys can use it.
i would pick up the guns and check them out for yourself but to me the mossberg is the better gun. both will shoot but one has better features.
m590 blackwater is a beast or 590 spx