500:
Lighter Receiver
the magazine tube is open and exposed for easy loading
You can’t easily replace the spring and clean the mag well
Harder to strip and put back together
about $75-100 cheaper
you can’t change the mag capacity easily
Safety on top
Action release at the rear of the trigger
Trigger is heavier
870:
Heavier Receiver
You have to push through the carrier to get to the magazine tube
Springs are easily replaced
easy to strip and put back together
more expensive than a Mossberg 500
Mag Capacity is easily changed (With a Dremel)
Safety on the side
Action release at the front of the trigger guard
Trigger is lighter