Were I the one choosing, I would choose whoever would do the best job, without going power hungry. Ban history would obviously play a factor in determining ones likelihood of straying from the path of ninjahood, but inversely having a ban history would give an individual insight on to what it actually feels like being banned. This could lead to them dishing out banishment more fairly. Ultimately, it just comes down to who the person is. Simplifying it too one factor, such as number of bans, seems counter-productive.