I've had a lot of 870's, many. I have had 3 Mossburg 500's that I bought in pawn shops for $135 (used) with screw in chokes. I have yet to have any problems with the Mossburgs.
870's are probably twice the price of the Mossburgs. For guys that live on a tight budget, price alone will make a choice for them.
For me, the placement of the safety on the Mossburg is much more user friendly than the placement of the safety on the trigger guard, and I have been shooting Rem 870's, 1100's, 11/87's, and Brownings all my life.
When it comes to something like this, pride of ownership comes into play, and to many guys, it is just below their dignity to shoot a Mossburg.
I shot doves in Az. with a 410 pump in a Mossburg 500 and never had any problems after thousands of shells fired. I had a truck gun that was a Mossburg 500 12ga-3 inch with screw in chokes that I really beat the heck out of, and it never failed to feed.
I do believe that the military and much of the police are using Mossburg 500's.
There is nothing wrong with the 870's and neither the 500's or 835's, they are all good guns. I consider the Browning BPS the cadilac of the pumps that I have used, and consider it a much better gun than the Win Model 12.