Well...sort of depends on the situation that you anticipate. A few more mags might not matter. Seems to me that the majority of attacks are by a single attacker. Not always, usually.
In some situations the black powder pistols might even have an advantage. They make lots of noise, especially on the business end. Some attackers may have never heard them. Heck, if you're close, maybe indoors, you might even momentarily stun your attacker with all that fire, smoke and boom, even if you miss. They might even drop their weapon or run. Might give you a chance to move (you better move) and shoot again if you must. Smoke might confuse him (or her) into shooting at the smoke and not you...if you move.
Anyway if I'm forced into it, and could not get away...... I would want as few shots as possible for lots of reasons. When the LEO gets there, they won't know who attacked who, and whoever is holding a gun is a shooter. I would never want to be in a protracted gun fight.
With BP, I would be more worried about losing a cap than only having six shots. Concealed carry might be hard with a BP cap and ball, and I think I would prefer concealed to open carry for self defense. So yeah, it would probably be a small auto.
But with BP, I'll bet most attackers today would think they were being fired on with a cannon.