Ellipses should only be used if a character’s speech is trailing off (and that should only be used once in a long while) or if you’re quoting something and omitting part of the quotation.
Dashes are used to set off a portion of a sentence that adds information but isn’t crucial to the meaning of the sentence.
Check out this Resource for Writers! Does grammar have you baffled? Would you like a simple guide that’s easy to understand? Check out Woe is I: The Grammarphobe’s Guide to Better English in Plain English by Patricia T. O’Conner. And look at this! She has a blog!
See you next week for the Grammar Owl’s take on a grammar/word use question! If you have a question you’d like the Owl to tackle, you can leave it in the comments, or send an to the Grammar Owl at mail (at) flubs2fixes (dot) com.