"What about you?" versus "How about you?"

As a native English speaker, I feel they have different implicit meanings. Without knowing the context, this is how I interpret them:

• I'm going straight home after work. How about you?

  I'm going straight home after work, which I assume you are also doing, would you like to accompany me as far as it is possible to travel together? Or, can I give you a lift?

• I'm going straight home after work. What about you?

  I am going straight home after work. You are not invited to join me, and I am not interested in doing anything with you, but I expect you will not be going straight home and I am curious where you are going instead.

John Lawler put it excellently in his comment above: "'What about you?' requests a statement about you in general, while 'How about you?' requests a response about your manner, means, or condition."

From my point of view, if the difference between what about and how about in general is slight, the difference between what about you and how about you is even slighter. They are certainly interchangeable, as you mentioned, but I would go so far as to say that their common usages are semantically indistinguishable.

In point of usage, Ngrams shows a slight preference for What about you:

COCA shows 770 instances of how about you, the vast majority of which are in the proper context (a few are in the form of how about you do so-and-so), and 1002 of what about you, all of which that I saw were in this context. BNC has 78 versus 202, an even more marked difference.

^{Disclaimer regarding Ngram chart: see this meta post for important info on this internet resource.}