Should I use ... or —?
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.
Source: https://nhwn.wordpress.com/2011/09/13/grammar-ease-ellipsis-versus-the-em-dash/