I believe a Remington 870 is superior to any Mossberg pump. The Mossberg's seem more loose fitting and sloppy. Not sure if still the case, but the military carried 870's in the past.
I own two 870's. Have a fully rifled barrel with cantilever scope mount made by Hastings. Slugs are accurate past 100 yards without issue. I do not like shooting slugs from the bench anymore. (Sighting in) So, I went to a muzzleloader.