Burton's bats were in part a German Expressionism. Like Burton said, they were going back to the silent movie acting with presence and stare, with which Keaton did wonders. Burton's movies were art movies, something that we should keep in mind. It was all about art, German Expressionism and Gothic narrative
As I said before, picking one or the other isnt picking an actor, its just picking an approach. Keaton was stunningly perfect for the Gothic expressionist presence he was suppose to imitate, like in Shreck's Nosferatu (another Expressionist Gothic story in which characters worked with presence and eyes) which was cited as influence
Bale on the other hand sold the angry, dedicated and skilled Ninja who also had need for regular' man's life perfectly for the conventional, "regular" movie portrayal. They both fit their roles and world's perfect to a T, and thats exactly the view that the great late Heath Ledger shared about Burton's and Nolan's world. They were both perfect for their own genre