I would advise you not to go for St. Mary's, that is on the other side of the city, so is a fair walk from lectures, plus it's at the top of the steepest hill in Bangor which makes getting back to the hall very difficult.
Going for the quiet hall is an option, but I was told that priority is given to those with special circumstances first, then other students. M block of Y Borth is not that big - it has 48 rooms, and given the amount of applicants for rooms, it'll be good luck if you get it, but there's every chance you might get them.