There is a slight difference. "How about?" is often used with an action as in: For example; How about going for a walk? In this sentence, there is a suggestion as what do you thing about going for a walk? 'What about?' is often used when someone is asking for a response to a choice as in: For example; What about the green one? Thats much pretty. In this case, you are asking for an opinion for one thing from a number of different things.

"What about..." is when you are asking about someone or something......for example, "What about John, is he coming with us?" or "What about the cookies? Are they ready?" "How about..." could be used the same way..."How about John, is he coming?" But, you can use "how about" when you are asking someone if they would prefer something over another. For example, "You don't want cookies? How about ice cream?" You are asking them if they want ice cream in stead. "How about going to the park?" It means, "Would you rather (or would you like to) go to the park?" The two phrases are very similar

What's the difference between what and how?