1. 'How about..?' and 'What about..?' can be used interchangeably when you are making a suggestion: What should we have for dinner? How about pizza? What should we have for dinner? What about pizza? Additionally, 'What about...?' can be used in the sense of 'Wait a moment, haven't you forgotten ...?' : 2. 'Then we can all walk down to the beach for a picnic.' 'What about grandma? She can't walk that far.' 'Oh, of course. Silly me. Well, let's go to a restaurant instead.' You can't use 'How about..?' in this sense.

In English spoken language people use the terms - perhaps incorrectly - interchangeably.