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?
Some spakers will benifit from their distance to the corners. i.e.better bass reinforcement. The long wall won't give you this unless you want your speakers 15 ft. appart. Then you would have to be sitting way back. It requires some fiddling to avoid the rear wall bass hump.Just be sitting about 1 ft. from the back wall.
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.
I tend to like long wall if speakers must be close to rear wall and short wall if I can pull them out 4-5 ft. from the rear wall.
What works best will be determined by the type of speakers that you are using and how you end up having to place them due to traffic patterns & furniture. If this is a dedicated audio room, have some fun, do some learning and experiment : ) Sean