Not much. And they overlap. No definite rules. Is seems to me that "How about" means "would you like to?" and suggests a simple "yes" or "no" answer. "What about" CAN mean that, too, but it seems to me to invite suggestions of alternatives. And it can also mean "Have you taken this into account in planning?" Q: "How about going out for a cheesesteak?" A: "Sounds great, I'm up for that." or "No thanks, I'm watching my weight." Q: "What about a cheesesteak?" A: "I'd rather have a meatball sub." Q: "How about all going out for cheesesteaks?" A: "What about John, he's vegan. Maybe Chipotle would work for all of us."