There is always someone with a Mossberg knocking the 870 Express. The fact is that both the Mossberg and the 870 Express are good guns. The only knock on the Express is that cheap ammo sticks in the chamber unless you polish it out with some still wool wrapped around a dowel chucked in a drill when it is out of the box. Some swap the mim exractor out for a machined extractor.
As for the Mossberg, the top tang safety is no good if you want a tactical pistol grip. They are also known to develop problem with the latches so that when loading rounds are dropped. Some hate the for end rattle of the Mossberg but most don't mind it much. That safety is plastic and prone to break. If it does break, you cannot fire the gun. Pretty easily fixed, but fix it if you get a Mossberg. (Not sure but the more expensive military model may have a metal safety).
Ask yourself why most Chinese made copies attempt to clone the 870 and not the Mossberg.
Both are good self defense guns, but I am beginning to believe a handgun is better. Best is a handgun and a shotgun.
As for gauge, I am happy with my 12, but some experts suggest a 20 so that the entire family can control it. For the same reason a semi auto might be a better idea then the pump. (The gas system of a semi auto sucks up a lot of recoil, but be careful. Not all auto loaders are gas operated, while some are recoil operated.) Not sure, but I believe Mas Ayoob recommends a youth size gas operated semi 20 gauge for a family home defense shotgun, but again I believe he prefers the handgun over all. Read
this by Ayoob
I shoot 12 gauge shells that I reload to emulate a 20, but that is for target shooting. One benefit is that the weight of the heavy 12 gauge target gun sucks up much of the recoil and that is true of the 20 type load, but I would not want to use reloads for defense.
Choice is also dictated to a great degree by your circumstances; rural or city, single family home or apartment, family across the house from master bed room.
There are a lot of accessories for either the Mossberg or the 870.
Besides the interchangeability or lack thereof of parts between an 870 and a clone, consider barrels. There a lot of barrels for both the Mossberg and the 870, but the 870 barrels usually cannot be fit to a clone without some kind of adapter that is usually made out of pvc pipe or conduit. Also, the barrels that fit a Mossberg 500 aren't going to fit the model with the larger capacity magazine.
I think what I am saying is you are doing the right thing by educating yourself before buying. The best gun and ammunition are the ones tailored for your needs.