My DD(7) prefers Elsa because she thinks Anna isn't too smart.

... which gives me hopes for her when she is a teen. Elsa's ice power is just a bonus. I think most littles probably are impressed by the really pretty blue sparkly dress and the ice power though.
Snow White has never been DD's favorite either. She likes her, but I've heard her referred to as the "dumb" Princess a few times at our house. According to DD, who on earth would open the door and take an apple from a scary looking lady, after the dwarfs had warned her multiple times not to open the door to anyone and she knew someone was trying to kill her.