I'm not a big fan of the Remington 870 simply because of the controls. We have both Mossberg 500's and Remington 870's in the Military and I prefer the Mossberg.
In order to work a Remington, the finger has to come off the trigger, engage the awkward and uncomfortable action release in front of the trigger guard, disengage the safety and pull the trigger.
On a Mossberg the thumb actuates the safety, and you can easily engage the action release with the middle finger, never taking your finger off the trigger. These controls are more "natural" to use.
The Remington is cheaper to upgrade and put a larger magazine on.
The Mossberg uses plastic trigger assembly and safety. You can upgrade these parts buying the police or military versions, however plastic ain't metal. I have many thousands of round through my Mossberg 500 and it hasn't failed. Just saying plastic is plastic.