You propose them both, and hope they come back with rental concession. But of course, only if you really are willing to move.
This concept I learned from a lawyer who did something for me unrelated. When you really don't have a legal leg to stand on, then you just try to appeal to the "good side" of the other person in hopes they do something for you.
In this case, you don't seem to have a legal leg, so going in demanding a certain rent reduction may offend the other side. And once they are offended, they'll just say "no", and you'll never get what you want.
So going in from an angle of, "this was a big reason why we signed the lease here" and that it's so important to you that should they have an open top floor unit, you'll move immediately, conveys the impression that this really IS important to you. What you are actually hoping for is that they don't, or it's too difficult logistically/bookkeeping that they rather just offer you some concession.
Now if they do offer you a new unit, then I would move, but perhaps ask for some moving related expenses or something.
Of course, if moving is off the table for you due to time/hassle, etc, then I wouldn't ask for that.