Joan was really the glamour girl who took a modest talent and worked like hell to better herself. She radiated star quality on screen at first but not much else. She put in the hours - grew with each film - learned by doing tons of radio shows. By 1937, she was pretty damn good. There are fine performances by her - as well as some pretty bad ones.
Bette Davis has pure natural talent but was hampered at the start by lack of the visceral impact and good looks on screen of a Crawford. She also worked like hell - to perfect how she looked and was lit. One could say that no one before or since has bettered the work Davis did in 1939 alone, including Davis. Also, All about Eve, is as good as later Davis got.
"There's no damn business like show business - you have to smile to keep from throwing up." - Billie Holiday