Messenger.Broadcast VS GetComponent

Do you mean BroadcastMessage? GetComponent is faster and more versatile: you can call any function, or access any variables in the target script. On the other hand, the target script must be known at compile time, what can be a problem if it was written in a different language.
BroadcastMessage takes more time, you can pass only a single argument and receive no return values, but it's the better option when you must call the same function in different scripts. Good examples are the function "Fire", that can be called in the current weapon even if each type uses a different control script, and "ApplyDamage", which also may exist in an enemy object, or a wall, or a tank, or the player etc.