HELP Which wall of new room to put system on?

I'd put the speakers on the short height long wall. The upward slope on the ceiling may help first order reflections since it would be more of a tangential interface. Long wall placement also gets you away from the side walls which delays the first reflections thereby helping that problem some as well. If the seat/couch is close to the back wall adsorption directly behind your head (i.e big pillows) helps a ton. My speakers are long wall positioned and the sound stage is significantly wider and more open than a short wall set up.

If it is a new room, and WAF is not the determining factor, why not try it several ways and see what you think. One setup that I have heard, and which is rarely if ever mentioned is a diagonal arangement, where the speakers are not against any one wall. It sure confuses those pesky room modes.

Depending on how tall your speakers are, I'd go with wall with the sloping ceiling. You should also consider where you're going to place your equipment rack/stand. If you look at the virtual systems section, you see that over 95% of them have equipment between the speakers. IMO, you want to keep source equipment away from the speakers, and big racks and video screens do not enhanse imaging.
Having your main front speakers 5' off the wall will be great for imaging, but you'll also have some peaks and dips
in bass response to deal with.

Piezo, both long walls (20') are the same height.

The room I have now is 20' x 13'8" with no angles to worry about. I tried the speakers on the short wall and the long wall. On the long wall, I got better horizontal imaging but there didn't seem to be any depth to the soundstage. On the short wall, I was able to pull the speakers further off the wall since I could move my listening point further back. This really gave good depth to my soundstage. I'm assuming that I will probably get the same results in the new room and end up putting the speakers on the short wall again.

Kana813, the equipment will be between the speakers. The equipment is a 57" widescreen HDTV with a 5 shelf rack on each side. (All of the equipment is short enough to go against the 5'4" wall.) So, the equipment will be against the wall and the speakers will be pulled about 5' off the wall. At least, that is how it sounds best in the room I have now.

Keep the advice coming!!!!!!

Try them both and let us know!