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.

I can't add anything to Jessica's explanation, but I can't resist adding a cultural note. There's a nice old song entitled "How About You?" https://www.youtube.com/watch?v=jgoG5o7Jsk4 I like New York in June, how about you? I like a Gershwin tune, how about you? I love a fireside when a storm is due I like potato chips, moonlight, motor trips, how about you? I'm mad about good books, can't get my fill And James Durantes looks give me a thrill Holding hands in the movie show When all the lights are low may not be new But I like it, how about you?