Who/whom

This sign is displayed in Hot Springs, Arkansas. Old-school diagramming helps with understanding this sentence. 

Diagramming purists will find errors with my diagramming form. I beg for mercy since I didn't learn diagramming until college. 

When working with questions, I find it easier to transpose them into declarative sentences. In this case, you will choose to be who. The subject is you. The verb is will choose. You will choose what? The answer is the direct object to be.  To be is a verbal. This means that while it is not a verb, it has qualities of a verb. One of those qualities is that it can have a direct object of its own. Ask, "to be what?" in order to find the direct object. One needs an objective pronoun to fill the position of direct object. The pronoun who is a subject pronoun. The pronoun whom is an object pronoun. Whom is the correct choice here. I do agree that whom would sound pretentious. The Arkansas School for Math and Science should embrace pretentious.