What about? -vs- How about? ...and Will -vs- Shall
This issue has always confused me:
Hi! How are you?
-I'm great, thank you. What about you?
-I'm great, thank you. How about you?
-I'm great, thank you. And you?
What's the more natural answer? Are "how about" and "what about" interchangeable?
I can't understand the difference very well, I always seem to choose the incorrect one.
How was your day?
-It wasn't bad! what about yours?
-It wasn't bad! how about yours?
-It wasn't bad! and yours?
(same problem...)
-What about going out tonight?
-How about going out tonight?
(same problem...)
Any help appreciated.
21 Answers
Hi! How are you?
-I'm great, thank you. What about you?
-I'm great, thank you. How about you?
-I'm great, thank you. And you?
Hi Cogu. You will hear all of the above used more or less interchangeably. My English ear, interprets the replies very very slightly differently:
"I'm great, thank you. What about you?" This very slightly places more of an emphasis in returning the question. So what you are saying is...."I'm great, thank you What about you? ie forget me, tell me about how you are!
"I'm great thank you, how about you?" Same as the above, but very slightly less emphasis on how the other person is. More of a equal - How am I / How are you.
"I'm great, thank you. And you?" More of an emphasis on how I am. I am almost throwing back a "and you" to be polite, I'm not really interested.
Same answers for "How was your day?"
What about going out tonight?
How about going out tonight?
How about going out tonight, I interpret as more of a question/request and sounds a little more natural than the other.
These are how I see the differences Cogu. In reality these are very very subtle and I don't think anybody would think it strange if you used either the 'how' or 'what'.
Any other native English speakers please feel free to comment on my thoughts.
That's clear, but what about the third example?
As others have said, "Would you like to go out tonight?" is much more common, especially if you don't know the person or this is the beginning of this part of the conversation. "How about going out with me tonight?" is more familiar. In the USA, "Shall we go out tonight?" would sound really weird. "Shall" is reserved for formal situations or for set phrases like "Shall we dance?". If I were talking to my wife, I would say "You wanna go out tonight?" or "Whaddya wanna do tonight?".
And you are correct that it is not so simple and what I indicated above is not a rule.
"What would you like to do tonight?" (to someone you know well)
"How about going to a movie?" is the correct choice.
"What about going to a movie?" is more like rejecting the original suggestion (in a longer conversation) and proposing another one, or refering to a past agreement that is being broken--although "What happened to going to a movie?" would probably be more common in that case.
And of course, "How would you like to do tonight?" sounds weird except in really specific circumstances in which it means something else.
Hola Cogumela 
Hi! How are you?
-I'm great, thank you. What about you?
-I'm great, thank you. How about you?
-I'm great, thank you. And you?
The most natural answers would be How about you? and And you?. Those two are pretty much interchangeable. I wouldn't use What about you? although it would be understood. As a general note, you want to use the same "question word" (i.e., How) that the person you're talking to used.
How was your day?
-It wasn't bad! what about yours?
-It wasn't bad! how about yours?
-It wasn't bad! and yours?
Same Answer. How about yours? and And yours? are perfectly acceptable, and What about yours? would be understood, though not a first choice.
-What about going out tonight?
-How about going out tonight?
Personally, I would probably only use the "How" one (or How about we go out tonight?). The first one just seems awkward to me.
What about going out tonight? could be understood as "what about the idea of going out tonight" vs."Let's go out tonight". For example, two sisters could be at home talking about going to the movies that night and their mom could say What about "going out tonight"? as in, "Cómo que uds. salen esta noche?"/"What [is this I hear] about going out tonight? Your chores aren't even done!"
In general, when it comes to suggestions, use How about...? for actions and What about..? for people, places, or thing (nouns). I don't know if that makes sense. Here some examples:
*How about staying in?
*How about catching that new movie?
*How about we go get some Chinese (food)?
*What about this book? It's only $5.99!
*What about the Rufus (the dog)? Won't he be lonely?
*What about Goodie's Chinese Carry-out? They're open.
Of course, there are exceptions 
and preferences. And of course, you can skip all this hoopla and just use Why don't..? (or Why not..?)
*Why don't we stay in?
*Why don't we catch that new movie?
*Why don't we go get some Chinese?
*Why don't get this book? ***Why not this book?***
*Why don't we go with Goodie's? (i.e. choose Goodie's) ***Why not Goodies?***
...Errrrk. Wait, that isn't exactly faultless either. Not to mention, I'm starting to confuse myself ;P But I sure hope you got some clarity and some new ideas on ways to express... stuff. Sorry for the book! 
Open to corrections
In general, when it comes to suggestions, use How about...? for actions and What about..? for people, places, or thing (nouns). I don't know if that makes sense.---> It makes perfect sense! Thank you!
Sorry. It doesn't make sense to me at all.
Where do you want to go?
How about Paco's?
"What about Paco's?" doesn't work here.
What do you want on our pizza?
How about peperoni and mushrooms?
"What about peperoni and mushrooms?" doesn't work here.
I guess that in these examples, "How about. . ." makes a suggestion, while "What about. . ." asks a question about a suggestion that had previouly been made. This distinction is not universal.
What do you want on the pizza?
How about peperoni?
What about mushrooms? (You always have mushrooms with peperoni!)
How about ham and pineapple? (You know I hate peperoni!)
With will and shall, I guess at times they can be interchangeable. Not always, though.
'Shall' can be sorta like 'will' and 'should' a times and also, 'shall' has more of a permission meaning, like someone's asking permission or an opinion.
por ejemplo "Shall I go to the store?"
In that case, you could probably substitute 'should' for 'shall.'
Most of the time, you will probably hear 'will' and 'shall'. 'Will' more likely be used if someone's trying to be really formal. Also, 'shall' is mainly used only in the first person plural and singular.
I hope I'm not confusing you more. 
Annierats,
I like your "Shall we go out tonight?" However, imagine the following conversation:
-Shall we go out tonight?
-Yes, that's a good idea. We could go to the cinema!
-Cinema? Hmm.. I don't fancy going to the cinema tonight. How about going dancing? / What about going dancing? or still, "shall we go dancing?"
How was your day?
-It wasn't bad! what about yours?
-It wasn't bad! how about yours?
Would you ever say "What was your day?"
. . .and "What are you?" means something completely different.
If someone asked you in Spanish "¿Para qué XXX?" would you answer "Porque YYY."?
All very confusing Cogu , you should come to OZ we don't use either expression,
we just say ,-G'day howya goin? Oh G'day great ,owr you . That is it !
Then further tete a tete, Gwanna go fr a dance or a drink or both in the back o me
Ute ? It is all cultural and it differs all over the English speaking world , like,
Kiwi will say,- G'day bro who was that sheep I saw you with last night.
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 and motor trips,
how about you?
I'm mad about good books,
can't get my fill,
and Franklin Roos'velt's looks
give me a thrill.
Holding hands at the movie show,
when all the lights are low
may not be new,
but I like it,
how about you?
Also for how about and what about.
How about is used more in making suggestions. What about can have more of a meaning of "What's the problem with? or What's wrong with?" or someone might say it when they know the general idea someone is/was talking about, but don't know exact details.
example: someone might hear some talking about 'Billy' but that's all they know, so they might say, "What about Billy?"
example 2: someone might say, "I don't like that store." someone else might say, "What about it don't you like?"
What about can also be used when somebody thinks someone else was forgotten or purposefully excluded, and they want to know why .
example 3: Person "I'm bringing you and Bob." then Bob might say, "What about John?" "Aren't you going to bring him too?"
These are just my examples. I'm sure other people have some others and better ones.
Right, we have run out of space on the comments:
@ Stadt: The use of ' wanna' is very bad and sloppy English, in Hull, or in the Westcountry. The grammar does not change with the region. The accent does, but not the grammar! Shall and will are just normal, polite words for asking questions. I look forward to seeing you, I shall introduce you to my mother-in-law. She decread that tulips are a 'vulgar' flower and so are most modern roses... That's when I first knew her, she is a little mellower these days. A little. In her presence, I think, you won't utter the word wanna, it will just not form itself in your mouth, I guarantee it.
I can't se that you could choose the wrong one, I think they are interchangeable. For going out tonight;
Shall we go out tonight? may sound better, as a simple option, but you could say, How about going out tonight?
Cogumela, what fun! Shall we go walking in the woods together or shall we go swimming?
The following is from the internet.
There is a slight difference that I could illustrate. 'How about?' is often used with an action as in: How about going for a walk? This suggests: What do you think about going for a walk?
'What about?' is often used when someone is asking for a response to a choice as in: What about the blue one? That looks attractive. This suggests you are asking for an opinion for one thing from a number of different things.
Hi! How are you?
-I'm great, thank you. What about you?
In my opinion, your option is "what about you". And the choices are good to bad and include what you might feel in between.
En mi opinión, tu opción es ¨what about you¨. Y las opciones son buenas a malas e incluyen lo que se puede sentir en el medio.
Corrections to my Spanish are appreciated.
(Btw, I can't understand what you mean with "the choices are good to bad")
The question is “Hi! How are you?”
Answer: I am fine. (good)
Answer: I feel terrible. (bad)
Answer: I feel tired. (Because of being tired, a person does not feel good or bad but they feel somewhere in between “good to bad”)
I hope this helps!
How about vs. what about.
Many people use them interchangeably and without distinction.
For me, how about is more general and relates more to emotions and physical state of being while what about is asking for specifics and actions.
Hello, how are you? I'm fine. How are you? I'm fine as well.
Yesterday I went to the dentist. What about you? (what did you do?)
