First, ANYTHING you use MUST be 100% reliable in your gun.
Then, it boils down IMO to what distance you are going to be using it.
0 to 10 ft.....doesn't much matter what you use. It will barely have left the shotcup if at all so 1-1 1/2 oz of lead is pretty much the same.
10ft to 25yds.....Buckshot or Slugs. Some prefer 000, some 00, some #4. I kind of like 12ball 2 3/4" 00 loads as they don't kick as hard as the 15 ball 3" and will penetrate well out to the 25yd limit. #4 might be a better choice if on the short end of the distance with more pellets per shell but that is a subject of MUCH discussion. Slugs work well here too, but normally recoil pretty stoutly. Shot spread to help poor marksmanship isn't really a valid concern as my 18" cyl bore guns normally will pattern 00 in about a 24" circle at 25yds so you must aim well to hit.
25-100yds.....Slugs. Only. But, it surely can be argued that a rifle would be a much better proposition for long range shooting. And, what kind of 'self defense' are we talking about when shooting at 100yds?
During 'Crowd Control' training in the service, we were taught to fire the 00 several yards short of the malcontents to spread the charge and hit many of them in the legs. Happily, I never got the chance to try it and still wonder if that really was an effective method. But, if that tactic is on the menu, then 00 is preferred.
Some will mention the 'overpenetration' of slugs and buckshot, but the 'Box o'Truth' showed pretty good penetration for birdshot through apartment type walls too.
Home defense is 00 Buck for me.