Will you or won't you?

Lesson Topic: Future Tense Using "Will" And "Be Going To" 
Question:  You are going out for lunch.  You hear your boss saying that his shirt is ready at the dry cleaner's.  You are a really faithful, helpful, and loyal employee.  What do you say?

1. I'm going to pick it up for you.
2. I'll pick it up for you.
3. Pick it up yourself!  You, lazy bum!

C is a great answer if you want to get fired.  If you don't want to get fired, we strongly recommend one of the others.
If you chose not to get fired, you are considering A and B.  Which one is possible?  Which one is best?  Which one is usually said by a native speaker? 
Both A and B are possible, but one is more common for a native speaker.  Though will and be going to mean almost the same thing and are interchangeable sometimes, there are some situations when one is used more than the other.  

Will 
Promises 

Johnny Mom, if I get 100 on my math test, will you take me out for ice cream? Mom Yes, I'll take you out for ice cream.  I promise.

Offers to help 

Patricia  I lost my ring Brian No problem.  I'll help you find it.

Request for help 

Peter I need to change the oil in my car.  Will you help me? William Of course, I'll help you.   Would is even more polite in requests for help. I need to change the oil in my car.  Are you going to help me? is considered impolite.

Predictions about the future 

Heather Some statisticians predict that there will be10 billion people in the world by 2050.   Melinda  I think they are right.  I heard there won't be enough food in the world for everyone. Will is often used in predictions, but be going to is also used and acceptable.

Refusal of things

Walter I can't open this bottle of wine. Sally Yeah.  The cork won't come out.

Be Going To
State a plan or intention 

Holly What are you going to do this summer? Mandy I am going to travel across Europe.  I am going to see everything from the Eiffel Tower to the Coliseum in Rome.