Everyone has their own idea of what a shotgun should be, and what qualifies it as best. Clearly the pre-Express vent-rib 870 Winmasters were very nice in appearance and operation. Now I'm a rifle guy primarily, so some will question my experience to even have an opinion on the subject of shotguns, but I do have a particular point of view. When I carry a shotgun, it's role is usually that of short range, fast shooting, powerful rifle. To me, reliability and ease of repair trumps all else. With that in mind, the Mossberg beats the competition due to its use of screws in place of rivets and stab locked components. Thus, anything that fails on a Mossberg shotgun can replaced by the owner with the use of house hold tools. That is a significant advantage when one lives hundreds of miles from the nearest gunsmith. The old 870s are a marvel of smooth operation, but a polished Mossberg isn't that far behind; when I press the slide release on my 590, while holding the cocked gun vertically, with a slight bump, the slide pretty much drops to the end of its travel.
The BPS is a great choice for the south-paw if you happen to be left handed, but there is a disadvantage. A bottom ejecting shotgun must be loaded into the magazine tube, whereas a shotgun with a right handed ejection port, can have a round simply dropped on top of the follower and pushed home. While a lefty might find that flinging empties across his line of sight can be disconcerting, he actually has the advantage when it comes to single loading the gun through the ejection port as he doesn't have to reach over the top of the receiver.