I keep my 870 loaded with the Remington Managed Recoil buckshot. It's 00 Buck, 8 pellets per shell. I like the reduced recoil, especially since (1) I might need to fire from my left shoulder, which is very recoil sensitive since I damaged it pretty badly in 2015, and (2) inside my house, I probably won'e be firing from a textbook stance.
I remember reading years ago that the key with respect to shot size and defensive use depends entirely on the range. At close rage (think across the bed) shot size is probably irrelevant. The load will barely spread, and you'll end up with a really messy single wound I once heard described as a rat hole (think about a rat chewing its way through something). Extend the range to the longer (longest in my home would be around 12 yards) and you start to get spread. With spread, you start to see lighter and smaller shot penetrate less, which negatively impacts stopping power. So larger, heavier pellets penetrate better, helping stopping power.