One of the big problems with a 20 guage for personal defense is finding suitable defense ammo for it.
Several manufacturers make the 2&3/4" #3 buckshot load. I consider that load suitable . If the local store doesn't carry them, there are on line sources.
Not the varity of loadings available for the 12ga. , but to me not a problem using that loading for HD. I do have a small supply of 3 inch #4 buckshot but I don't think they are making those any longer, and I don't keep anything but the #3 BS loading in my gun anyway, so for me ammo is not an issue.