Translation:I am the judge, and you are the lawyer.
in most cases they are interchangable. a quick googling confirmed that it's not only my feeling, but most hungarians consider "ön" more respectful, while "maga" makes the addressed party feel like you are being polite, because it is expected, but you really don't like him/her, or you consider yourself above them.
Ön for everyone you don't know if you want to show your respect. Maga with people older than you, exactly with old people.
They are interchangeable, however "maga" is very formal and is used to distance yourself from the person you are talking to.
That is kind of funny when a polite form can be impolite. But politeness always feels borderline impolite to me when it is not sincere. I usually hate ritualized fake behavior. Perfectly holy on the outside, but just playing a character.
I would assume using "maga" when e.g. getting your passport is ok, since neither of the involved parties want to become friends, it is strictly a one time business, but in personal life, you might start with "ön", but with openness to become afterwards friends or at at least good acquaintances. And in the workplace, I assume at least the boss usually will keep the "ön" status, but everyone else should be "te" in a short time.
How would this sentence change if it was supposed to say they are the lawyer¨, rather than ¨you are the lawyer¨?