have to or must

"You must leave immediately."
"You have to leave immediately."

If it were a boss telling a salesman that he would miss an important a meeting if he didn't leave immediately, which would you expect to hear, have to or must?

And if it were a person telling a friend whose mother had fallen ill to go and see her immediately, which would you expect to hear?

Lastly, if it were a security guard asking a noisy teenager or a loud-speaking fine-lady to leave library, which would you expect to hear?

I was tempted not to answer this question, because it really doesn't matter which expression is used. The expression on the face of the person making the request, is probably more important than the words used. Similarly, how the emphasis is placed on the words, and whether they are spoken quietly, or with quiet anger, is what is important.

In reality (at least in the US), I'd think you'd more likely hear the following in these cases:

<<If it were a boss telling a salesman that he would miss an important a meeting if he didn't leave immediately, which would you expect to hear, have to or must?>>

You['ve] gotta leave right now ...

<<And if it were a person telling a friend whose mother had fallen ill to go and see her immediately, which would you expect to hear?>>

You['d] better leave right now ...

<<Lastly, if it were a security guard asking a noisy teenager to leave library, which would you expect to hear?>>

... outta here right -- right now!