When UX Design is a matter of opinions and preferences, something has gone very wrong in the early stages of the process. Design creates solutions that perform a function. A function can and should be measured. Defining upfront the function (eg. complete a task under N seconds), how it will be measured (eg. analytics, survey) and the target number (eg. complete >50 task/hour) will spare everyone on the team from painful and unproductive conversations. Even when the adequate analytical tools are not available, defining function and metric will help shift the conversations from "I don't like the blue on that button" to "Is the visual hierarchy appropriate to draw users' attention to the primary CTA?".