powerslider,

Use hyphen.

M-dash with spaces before and after it means pause or separation: "The sun was rising --- a red, hot plate". M-dash without spaces means range: "Please make shure you complete excersize in Jan---Mar". There could be also numbers instead of months. It also depends on country, where you plan publishing your text. For example, russian typography took many things from french, including m-dash and quotation marks. Find older books that were published before era of personal computers, when uneducated people started to play typesetters without eye of the Editor ;-). I have never used n-dashes, only hyphens and m-dashes.