[STCrowley]

Hey, majid72:

My first instinct was to say that there isn't much of a difference between them. (I'm saying this about how I'd use them as a native speaker, not for the differences between the simple future and the future progressive.) But then I realized that there was a difference:

If I said "How long will you stay?" it would sound to me like we were talking about this for the first time, or like I maybe didn't think you knew how long you would stay. (Does this make sense? I'm saying I don't think you'd have a plan for how long to stay in the hotel.) If I say "How long will you be staying?" my feeling is that I know you have a plan--you've thought about this before--and now I want to know what the plan is.

I don't feel like this is a crystal clear explanation. If I can help in any way, let me know.
-Toby