English calls for the dash for trailing off -- a sentence that got started that did not get finished. Many, many reporters are using the ellipsis.
When the person uses but before he trails off, there is no comma before but because there is not an independent subject and verb after. When a person uses so before he trails off, there is a semicolon because it means "therefore."
...at the event but...
...at the event but --
...at the event; so...
...at the event; so --
Happy punctuating!
Margie