These two phrases are practically the same and can be used interchangeably. Normally, "how" and "what" are different. However, saying "what/how about you?" is very casual, so using either word will work. If you're speaking to someone professional, I think it would best to say something like "and yourself?" For example: Casual: A: What is your favorite color? B: Blue. How/What about you? Formal: A: How have you been lately? B: I have been well. And yourself?

I don't think there's any difference; they're very interchangeable.