If the Mossberg you are asking about is built on the 500 I would choose an 870 over it, but I would choose a 590 over the 870.
The difference is the magazine. I like to able to take the magazine apart for cleaning, and it is simply not possible to do yourself with a 500. Given a choice though between the 870 and the 590, I prefer the 590 thanks to the ejector that is held in by a screw (590) rather than a rivet (870). The addition of a Sidesaddle provides a screw in place of the trigger group pin, (don't over tighten it) and a longer screw to hold the ejector in place and attach the front of the Sidesaddle.