bié 别 vs. bú yào 不要

In my last (real) post there was a whole lot of "bu yao-ing" going on to tell someone not to do something. Well, now I'm going to ask a question that I feel I've never really gotten to the bottom of:What's the difference between "bié" 别 do something and "bú yào" 不要 do something? I've asked various informants over the years and gotten answers ranging from "there's no difference" to "I can't explain the difference" to "one of them is more formal" but disagreeing on which one is. So now, I shall attempt to employ the power of the blogosphere to find out whether there is a difference and what that difference is.To assist, I offer the following examples:

A. bié shuōhuà le 别说话了 = stop speaking / be quiet

B. bú yào shuōhuà le 不要说话了 = stop speaking / be quiet

Questions (for ya'll):1. Which one would an elementary teacher be more likely to say to a room full of noisy kids?2. Which one would a friend be most likely to say to another friend when hiding during a game of hide and seek?3. Which one would a mother say to comfort a crying daughter who keeps going on about the goldfish that just died?4. Can anyone think of a situation or example where "bié" 别 do something would be ok but "bú yào" 不要 do something wouldn't? Or vice versa?5. When in doubt, which one should I use?If you've got an idea, feel free to weigh in with your answers. Let's see just how muddy this water can get!