Make up your mind vs. Decide

The English language phrases "make up your mind" and "decide" (or "make a decision") have similar but subtly different meanings as I will describe using British English examples.

To make up your mind means to form an opinion. When you make up your mind you go from a state where you don't know what you want, to a state where you do know.

For example:
“I made up my mind on my 10th birthday to become a pilot.”
“I know all the dresses are lovely, but will you please make up your mind and choose one.”

Note it is also common to hear "make your mind up". This phrase on its own is an imperative urging someone to make a decision more quickly, for example a stereotype impatient husband waiting for their wife to choose an outfit for the evening.

To decide has a similar meaning and can be used in many of the same sentences and situations, but to make a decision is normally used about an action that will be taken, not about forming an opinion.

For example:
“I decided to become a priest.” "I made up my mind to become a priest."
Both of these are equally correct, as is "I made my mind up to become a priest."

But:
“I've made up my mind that I prefer yellow.” is correct, whereas you would not say "I've decided to prefer yellow." While this second option is grammatically correct, it is not natural.