IMO, ellipses indicate a "trailing off" of dialogue, and that's how I've seen them most used. Time-wise, it's a bit shorter than a beat.
CARRIE (CONT'D)
You'll find that special person
...one day...maybe. And then
you'll forget all about me.
Or, used in another way:
POV CAMERA - SHANE'S FACE
Mouth agape, humiliated.
JUSTIN (O.S.)
And hold it...hold it...and...
A "beat" is a dead stop; a total break in the dialogue. I usually (based on how it's used in scripts and then conveyed on film) think of a beat as around a 1 to 2 second pause.
Ellipses don't convey that stoppage and are, in my opinion, a weak device to use to indicate such.
Like I said, I hate, hate, hate using the word "beat." Seeing the word "beat" takes the reader out of the story and reminds him/her that he/she's reading a script, rather than being immersed in a story.
I prefer to create a beat with some sort of action. I usually don't use parentheticals to convey the "beat," but in the example I posted in post #4 it works and saves space (doesn't pad the pages with unnecessary white space).
For my writing, I think it's wasteful (and bad form) to break up a fluid dramatic moment in the pages with a bunch of hard returns and an action line when the information is better expressed as an "action beat" in parentheticals (again, I don't usually employ this method, but it is effective if used properly):
SHANE
Oh! Well thanks for clearing tha--
(it hits him;to Carrie)
From the gym?!
That is much better, in my opinion, than writing:
SHANE
Oh! Well thanks for clearing tha--
It finally hits him. He glares at Carrie.
SHANE
From the gym?!
*shrug*
Just some examples of how I do it. Everyone who's read my scripts (pro and non-pro) likes the way I write, so I'm gonna stick with what works for me.
Your mileage may vary.