I was reading the thread on em dashes, parentheses and commas and I was wondering if you had any thoughts on this subject.

I tend to break off a character’s thought or speech often in dialog. I was using ellipses and em dashes interchangeably and my editor asked me to choose one or the other.

In general, is there a rule on the use of em dashes vs. ellipses with an unfinished statement or thought, especially in dialog?

What about in the middle of a sentence? Can ellipses be used in the same way an em dash is used to separate clauses?

Thank you, AW grammarians.