In British english: 'How about...?' is slightly more Passive / Open Voice or relaxed than 'What about...?, which is slightly more directed, formal or a commanding; but it is very subtle and there are no hard and fast rules. More emphasis (either way) would be communicated by the contextual cues, tone of voice, volume of voice and body language. Hope that helps. This is a typical example of how my sessions with my Advanced students might develop such ideas often whilst discussing art & literature, history or geoplitics, mountains or relationships. Content informs the style and nuance. .

Use “How About?” To Suggest An Action And To “Open” Possibilities: And Use “What About?” To Mention An Objection Or A Potential Problem:

Both mean the same they are just two different ways of saying the same thing. Some dictionaries say there is a nuance but in everyday life there really isn't