It really makes no difference. I shoot 1oz loads out of my .28g, and 1 1/8 or 1 1/4oz loads out of the 16, 20, and 12g all at about the same relative velocity.or pretty close to it.
The main difference is in the size of the reciever and weight therefore of the shotgun.
Most 16’s and 20’s are built on a much smaller and lighter frame than 12g’s making them easier to handle for smaller framed men, women and kids.
Recoil for all of them is about the same with similar weights of shot and velocity on the same frame with equal barrel lengths.
For self defense I’m extremely partial to the Remington 870 tactical as I grew up shooting 870’s from about 1970 until present. It is the most dependable and proven repeating firearm ever produced.