I also had personally favored Michelangelo's David before I had researched Donatello's.  Even though Michelangelo's David still seems to me more appealing, I feel that without the influence of Donatello's David, Michelangelo wouldn't have created such a magnificent, powerful depiction of the biblical hero.  I think Donatello was very innovative in his time, well enough to test the waters of society for them to accept or at least acknowledge the nature and the beauty of the human body, thus carving the way for artists such as Michelangelo to also bring about such intricate detail with the human body of David.

I believe that Donatello probably believed they weren't painted, just like Michelangelo.  However, I believe that Donatello focused on bringing back the bronze casting that was used in antiquity to further emphasize his admiration of Humanism and Classical influences in art form.