The 870 gives you 7 rounds, the Mossberg gives you 8.
If you ever need more than 2-3 for home defense purposes, 7 versus 8 rounds won't make much difference. Besides, it's more weight further out that makes the weapon that much harder to handle.
I love guns and I own a few. But exactly what kind of marauding zombies does one need to expect at their door to make significant difference between 7 or 8 shots.
What exactly do you have in your home that is going to make these zombies attack you.
I know they go for brains and all....
(Really, I am kidding. Kind of....)
Having actually been in a gun fight, I would opine if you are on round 8, you are going to die.