I'd rather see a sequel than a prequel, probably in any situation. AT least then you can extend and expand the story; but a prequel is an extended flashback to which we already know the ending.
(That's very basic, of course.)
Knowing the ending doesn't mean you can't enjoy a film. It's not about the end, it's about the whole film. For example, I know the ending to The Godfather, but I'll still watch it.
The Departed left only a few characters alive to base a sequel on, and none of those characters were in the top 3 in importance in the movie.
However a prequel would give you much more to work with as far as the characters and that is what makes a story.
If a Soprano movie was made would you rather see a prequel to The Sopranos or a sequel? I'd prefer a prequel just because the ending was so good and what more can you do with the characters at this point?
It can be interesting to see how things got to a certain point, you don't need a surprise ending to make a film great.
