I would say depending on the size of your speakers, if they are small back the up to a short wall but if larger the wide wall will give you a more spacious sound. personnally I would go with the speakers aiming at the furthest wall. Especially if doing a home theatre.
Long v. short
I am trying to set up a new room, but cannot decide which wall sounds better for speaker placement. The room is 12.5 x 21 ft, and I've always used the short wall elsewhere by default. However, the long wall placement sounds different, but I have not yet decided if it is "better". I think I am hearing a wider stage, but it may be influenced by reflections immediately behind the sitting position, which must be against the wall due to the short width of the room. Do any of you have any generalized preferred set up for floor-standers, independent of speaker model and manufacturer?