There's no real difference. If any it is only slight. And in spoken English we use both. Sometimes I say what about you and other times how about you.
The only time I'd really pick one over the other is something like this:
'How about you?' - if someone asks 'how are you?' Id so fine, how about you? A how question followed by a reply with how.
'What about you?' - someone asks 'what are you up to today?' Me: I'm going to play football. What about you? A what question followed by the what response.
But in the end you can still use either.
I hope this helps.