Room size

Actually I have 7' ceilings. The way the room is configured the short wall isn't an option.

I prefer the long wall everytime in a medium size room. The long wall yields better midbass energy and a wider soundstage. Besides, sidewall reflections are never an issue providing you can keep things symetrical.
You will mostly likely need room treatment on the wall behind you to stop reflections. Tube Trap half rounds work perfectly for this.

7' ceilings are our enemies, I'd suggest treating that ceiling as much as possible with diffusion