They both are good. I dropped my Mossberg out of a treestand, and it survived the fall, didn't go off. That is a huge plus in my mind...but I didn't want to try it with the 870...I try not to drop my guns 30 feet, if possible.
I have a Saiga 12, which took a lot of gunsmithing on my part to get it working 100%, but now that it does, it is my favorite gun.
I have 2 870's and a Mossberg 500, I enjoy shooting all of them, I also have some hr single shots and wife has a hr partner pump, I have a rifles barrel on my mossy and use it for deer, I have a cut down 870 for hd, and a 28" barrel 870 that I use for bird/ squirrel and rabbit hunting, I have things I like and dislike about all of them, the 870 does seem smoother but they all go bang when I pull the trigger and they all function properly.