I'm sure it's based on location, but I don't hear "What about you?" very often.
These statements aren't grammatically correct in a traditional sense (The exact meaning is expressed when it's said), but are only said as a cultural expression.
(E.g. "Don't you dare." isn't the same as "Do you not dare?" in terms of cultural meaning, despite the fact they are shortened forms of each other, excluding the question mark.)
"What about you?" is asking for what exactly is important about 'you' when the 'about' is emphasized, but when no word is emphasized, it means "What's your answer, if I asked you the same thing?"
"How about you?" probably branched off the latter statement, as I'm not too sure on how to interpret it literally.
But they're both correct if you understand it perfectly fine, as I'm sure you'd consider it correct, as you're asking this question, rather than about slang.