That's a good question!
How about you? is to ask the same question again:
Where are you from? I'm from France. How about you? I'm from Germany
Do you like rock music? Yes. How about you? Yes.
What about you? is when it's not a simple question, eg, giving options about what to eat;
'Paul wants to go to get chicken wings, Mary wants pizza, Phil doesn't mind as long as it's not expensive. Tim wants to go somewhere with good beer... What about you?
I hope that helps! :)