I have 3 870's. One old 20 ga. Wingmaster, beautiful, smooth as silk a real joy. My 28 year son has laid claim to it and I gladly gave it up. I have 2 Express's, one 12 ga., 18 1/2" rifle sighted barrel, tricked out with Knoxx Spec Ops Stock, Choate corn cob fore end with Picatinny rail and SureFire flashlite, factory magazine extension and sling. I have taken a shotgun course with this one and it has been flawless through several thousand rounds of all stripes. The other one is a plain vanilla 12 ga. with walnut stock and a 20" modified choke barrel with tritium rifle sights. It has a sling, but no extension or anything else. It too has been flawless.
I have two buddies with Mossbergs, and I have shot them a bit. Nothing wrong with them, and I wouldn't feel under armed with one, but I just like the Remingtons better.
No real reason other than I know the Remingtons like my tongue knows my teeth, and if I have to use one for serious purposes, my hands just know what to do.
At the end of the day, it really is the Indian, not the arrow. If you are proficient with the Mossberg, then that is the one for you.
It's really all about which one you practice with enough to just automatically operate without any thought.