I actually had to look this one up, because I've never really thought about the differences before. After reading over other people's comments, I agree with what they've said. (http://english.stackexchange.com/questions/13659/how-about-vs-what-about) The two expressions can be used basically interchangeably. The only small difference is that "what about" can also be used to express doubt or contradiction. "We should go see a movie!" "What about the kids? We don't have a baby sitter." It was also suggested that "how about" usually expresses more firmness of opinion, and "what about" expresses openness to negotiation.