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.