Most shot patterns expand in diameter at nearly the same rate for the most part so the above is not really accurate...#4 Buckshot would be far better. It will do damage over a wider area.
Weight of gun being lighter, recoil will feel the same with a lower power shell with lesser payload. A full power load in 20 will be a pretty stout recoil compared to the full power load from a 12.A 20 gauge shotgun, whether it is loaded with #2, #3, or #4 Buckshot is going to be a very formidable self-defense weapon. In fact, it is probably the better gun for most homeowners to use, due to its lower recoil.
Brent