I've watched five of Bette Davis' movies and five of Joan's movies... and I have to say that while Bette Davis was more showy and emotional in her performances, I kind of like Joan's more low key/naturalistic acting a bit better.
Both actresses were wonderful, but comparing them is like comparing apples and oranges. Both are wonderful, but so different from one another.
In fact, I think Joan's more low key/subtle take in Whatever Happened to Baby Jane was a wise choice because the twist at the end was more of a surprise.