The efficacy of a short vs. a concept trailer in the pimping of a feature length screenplay?

Anecdotes or advice on one or the other?

Trailers are great because you get to include some of the best bits and set a mood really well but, though I'm sure they exist, I don't see many people submitting that type of thing to festivals.

Shorts on the other hand go to festivals constantly but, I'm curious people's stance on the method. Is it better to flesh out a scene from the feature into short form or to write something of a companion piece that merely shares the same characters and universe?

Insight is appreciated.

My script is in the Forgetting Sarah Marshall (content and comedy) neck of the woods, if people are curious or if it matters.