Here is an example from Murphy's grammar textbook:
You are in a restaurant. You and your friend are looking at the menu. Maybe your friend has decided what to have. You ask her/him.
You: What ...?
The correct answer is "What are you going to have?" My question is: the decision is being made at the moment of speaking, so shouldn't it be "What WILL you have?"
When choosing a dish from a menu, there is no real discernible difference between “What will you have?” and “What are you going to have?”
Both phrases are correct. Sometimes a restaurant customer already has an idea of what he wants to eat if he is familiar with the establishment. Sometimes a speaker wants to emphasize the element of intention, thus going to might be preferred. If both customers have been reading the menu for a couple of minutes, going to leans towards the intention or prediction aspect. And sometimes it's just quicker to say “What'll you have?” There's nothing to suggest impoliteness or inappropriateness about either form.
However, I'd say the will form is more common when replying: I'll have the spaghetti carbonara.
The question "what will you have ?" comes across as slightly more impatient and less friendly than "what are you going to have ?". It's a subtle difference, but "what will you have ?" demands a more immediate answer than "what are you going to have ?". I also imagine "what will you have ?" would be more acceptable and less impatient sounding in an American rather than in a British context.
