When do you need to decide? If it's soon, I'd say A. I don't operate on promises and would choose the school with the actual offer.
If there's no rush, it sounds like B is your preferred choice and I would wait it out until I got a written offer or the deadline had arrived.
If there's no rush, it sounds like B is your preferred choice and I would wait it out until I got a written offer or the deadline had arrived.