If I was asked this questions, I would ask what is the goal of the button within the scope of the application. Then I would ask each how what they want will help meet the goal or objective of that button and how it might perhaps better meet the objective than their opponent. 

Depending on the nature of the situation, I might enter the discussion with my own additional research on the topic to help provide guidance on the situation.

Here is a great youtube video helping to explain the Day in the Life of a Business Analyst - Decision Support