Shield Master merely requires you take the Attack action. TWF requires you make an attack (with the Attack action).The bolded is untrue. Please refer to the shield master debates for clarification.
Edit, re: X and Y babble:
It's not the difference between "If X, then Z" and "When X, Z." It's the difference between "When X, Z" and "When X+Y, Z." TWF simply has more prerequisites.
Last edited: