"How about" vs "What about"

In most cases, How about... and What about... are interchangeable when they are used in interrogation and suggestion. 
For example:
How about going to the zoo tomorrow? = What about going to the zoo tomorrow?

The similarity between them is like the semblance between "somehow" and "somewhat"
There is a lot of overlap between the two, but

"How about" when making a suggestion that I feel is