I have a Mossberg 590 and my dad has a Remington 870. I prefer the Mossberg, but a lot of it has to do with my specific situation (and may not apply to you).
Most importantly, Remington has implemented several "features" in the 870 (save the 870 Police and Marine Magnum) that prevent the installation of certain "tactical" accessories.
Late model (>1990) 870s have two dimples machined into the magazine tube. This will prevent the installation of an extended magazine tube. AFAIK, there is no federal law restricting the magazine capacity of a pump shotgun like the 870.
Conclusion: Stupid PC feature.
Solution: You can get rid of said dimples by using a drill or Dremel-type tool. Or you can buy a Mossberg 500 instead, which does not have this stupid feature.
Furthermore, if you want to install a TacStar sidesaddle on your 870 Express or Wingmaster, you will have to purchase a special shortened slide. Otherwise the factory slide will bang into the sidesaddle and prevent full cycling. Remington will not sell you the shortened slide unless you have an 870 Police.
Conclusion: Stupid PC move by Remington -- sell the short slides to everyone and I'll be happy.
Solution: Buy a Surefire Responder. It has enough clearance for the sidesaddle. But if you don't have the $200 to drop on a Responder, buy a Mossberg 500 instead.
1. The Remington 870 series shotguns only have one extractor. The Mossberg 500 series has two. Two is better than one.
2. I prefer the ambidextrous placement of the safety on the Mossberg 500. Lefties will like it even more. I find personally that the placement of the safety allows more instinctive disengagement. Not only that, it allows manipulation of the safety without moving your trigger finger from its indexed position along the triggerguard. The 870's cross-bolt type safety cannot do this.
3. With my small hands, I like the placement of the Mossberg's bolt release. Situated behind the triggerguard, it is easily actuated by the firing hand, again without moving your trigger finger from its indexed position. Maybe persons with longer fingers can reach the 870's bolt release without shifting the firing grip -- it's located on the front left side of the triggerguard.
Downsides to the Mossberg as compared to the 870 (that haven't been mentioned):
1. Some have reported problems with the plastic triggerguards and trigger groups on certain Mossberg shotguns. Solution: buy a 500 or 590 Military model, with the metal trigger group.
2. Certain models of the Surefire Responder cannot be installed on the 590 without grinding off the bayonet lug. Solution: Laser Products now has a new model (which I have not yet seen offered for sale) -- the 623A1 or something like that -- which does not require removal of the lug.
Justin T. Huang, Esq.
late of Kennett Square, Pennsylvania