I believe they can be used interchangeably, there is a lot of overlap between the two, but personally I would prefer to use;
"How about" when making a suggestion that I feel is best.
"What about" when I am less set on the idea and more willing to listen to other suggestions.
*What about can also express an objection, whereas how about does not.*
"How about going to a movie?"
- "I would love to, but what about the kids?" [meaning, we would have to arrange for their care.]