HELP Which wall of new room to put system on?

I'm probably moving my system into a new room. The room is about 15' wide x 20' long. I will put the system on one of the 15' walls. The room height is 8' except that one wall only goes to 5'4" and begins a gradual slant up to 8'. I would like to get everyone's advice about whether I should put the main system / front speakers on the wall with the slant or on the wall that goes straight up to 8' tall. I'm hoping that, in some way, this will be a sonic advantage. I'm just not sure if the slant should be behind the main front speakers or on the other end of the room from them. By the way, the main front speakers will probably sit about 5' off the wall.
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.
Good question. I've seen folks advocate having the speakers on the wall with the high ceiling, and that may be so, but I would be concerned that your seat under the sloping walls would put you in a position to pick up early reflections off the sloping part of the ceiling. I suppose much will depend on where you put your chair. If the ceiling just in front of it is 8' it probably wouldn't make much difference.

In my last home, which had a sloping ceiling as you describe, in a slightly smaller room, and with the speakers closer to the wall under the sloping ceiling, I could never get a great sound. I remedied this by putting the speakers on the long wall firing accross the sloping ceiling and it worked fine.

Best to keep an open mind on the possibilities and try all four walls.
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.
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!
I've had to contend with sloping ceilings in both my systems (work and home). I'm currently struggling with the same question at home and am inclined to offer up my experience so far, to see if it may help, and to see if I can get any advice of my own. In my case I'm locked in to a configuration already so my questions are limited to treating the room.

My work system works out wonderfully: it is a rather large space, approximately 20X30 feet, with sloping walls on both long sides. The system and speakers are against the the lower part of one of the long walls with the slope rising up above them (as if they would be in an amphitheatre). Soundstaging is wide and deep and it all works out splendidly there.

The home system is just the reverse: the listener is seated under the sloping wall on the low side. This room is closer in size to yours, being about 10' X 25'. The slope in my room goes from about 8 feet up to 11.5 feet. This configuration does not work as well with the bass being a bit boomy, and the soundstage seems compromised and lacking focus. I'm speculating that the ceiling is going to require some kind of treatment to kill that reflection. If I had the choice, because of my experience at work, I would start with trying the speakers at the low end of the slope rather than the high end. A TV screen in between is definitley a compromise to imaging in my experience too, but when listening to music I've found that just throwing a blanket over the screen least acoustically (certainly not a visual highlight though).

Good luck, and keep the posts coming as you try to figure it out.

Thanks to everyone for the comments. It turns out that my new room is probably going to be a normal rectangle, no slopes!!!