If you want to get really nerdy with it, I enjoy looking at superheroes with RPG characteristics.
Superman:
+Brawling
+Strength
-Magic Resistance
-Mental Attacks
Batman:
+Martial Arts
+Evasion/Stealth
-Direct Damage
-Low HP
Wonder Woman
+Melee Combat
+Defense
-Poison Resistance
-Piercing Damage
This doesn't necessarily create an argument for or against Diana being bulletproof, but it keeps characters in check somewhat by highlighting strengths and weaknesses that I think make them diverse in power sets and abilities.