I would recommend completing Expert Mode first so you get a feel for the unique pattern changes that the Expert Mode bosses have. You'll have to deal with all of the unique Expert AI patterns in Master Mode and they will do more damage to you overall. Many enemies from the beginning will be able to 1-2 shot you until you get some life crystals, it is a similar case at the beginning of hard mode even when you have 400 HP.
Even in Expert Mode, it is easy for pattern differences such as Plantera's tentacles and Duke Fishron's 3rd phase to take a player off guard. Knowing how to deal with those patterns before beginning Master Mode will help.