I came across this recently and curious as to which is correct. Here are a couple example sentences:
I wish you would…oh, never mind.
I wish you would—oh, never mind.
In the above usage, which is correct, an Ellipsis
… or an Em Dash
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An ellipsis is used in a quote to convey that the speaker trails off.
An em dash is used in a quote to convey that the speaker is cut off, even if the speaker cuts him/herself off, as is the case in your second example.