I recently read a review in a magazine called "Gun Tests" They are independent, and are supported completely on the money provided by subscribers. No advertisements or money coming in from manufactures trying to push their products. They reviewed the Mossberg and Remington 870 Pump shot guns... Although they were reviewing the 20 gauge models, I'm sure the results would be the same for 12 gauge.
They chose the 870 over the Mossberg. They rate guns with a rating of F-A+. I think they gave the 870 an A, and the Mossberg an A- or B+ (Cant remember)
Their reasoning was pretty simple, they 870 just felt better, and worked Just as well. Sure they Mossberg was nice, and worked perfectly. But the 870 also worked perfectly, and was just a little nicer in general.
I own an 870 12 gauge, 6+1 shot 18" barrel, all black. I love it. it's great to shoot clays with. and just fun shooting. And for home defense, I dont think it can be beat. It will shoot 2 and 3/4" shells, and 3" shells. I did alot of research before making up my mind. and I am very happy that I chose the 870 over the Mossberg.
In all reality, you'll be happy with either one. But if your really picky (like me) and like just a little extra style as opposed to "cool factor" you'll go with the 870.