I have both the Mossberg 590-A1, (standard finish), and the Remington 870 Police Marine Magnum. The Police Marine Magnum 870 is the better gun. I'm not saying or suggesting there is anything "wrong" with the 590-A1, there isn't.
But it doesn't compare. As was mentioned the 870 is all steel. The only thing Aluminum is the trigger assembly. (And that is only on the Police Marine Magnum). The regular Marine Magnum has a Polymer trigger assembly.
You can argue which is better, but I much prefer metal, as does the military. Which is why they specify it on the 590-A1. Along with a metal safety switch instead of plastic.
There seems to be some confusion about the Remington 870
Police Marine Magnum. The Order Number on the box MUST be 25047 for it to be a
Police Marine Magnum. If the Order Number is 25012 it is a Standard Marine Magnum.
The Police Marine Magnums are built in a different area of the plant, and receive a better degree of fit and finish. They also have milled extractors. The Standard Marine Magnum has a MIM extractor. That is not "bad". I have heard of very few MIM extractors failing on 870's.
The other difference with the Police Marine Magnum is it comes with adjustable
rifle sights. And a Butler Creek padded sling with the sling swivels sewn into the sling itself. It makes for a nice setup. They are more expensive than the Standard Marine Magnum. But they are the finest 870's on the market...... When you can find them.
There are usually a few available on Gun Broker and Guns America. But Remington only makes a few production runs a year of these. So you have to, "strike while the iron is hot", so to speak. They are among the finest pump guns obtainable. Just be SURE to check for the 25047 Order Number. Here is mine, along with my Mossberg 590-A1.