If these were my only options I would opt for the shotgun. I don't think anyone should underestimate the capablilites of a slug up to 75 yards even with open sights. Since both guns have open sights you will have a BIG advantage with the shotgun since you will have quicker follow-up shots if necessary. You didn't say if it was a pump, autoloader or single-shot, but either way you will reload much quicker than with any muzzleloader.