Side or back wall more important for speakers?

Try pulling your speakers out into the room as much as your bass frequencies allow (front wall reinforcement plays a big part of selecting this distance) and the place your speakers up to with in 18 inches of the wall with them toes in so that the axis of the speaker crosses well in front of your listening position. This toe in will kill most of the lst reflection issues from the side wall. Try it, it can work. Professionals frequently use this for set up in small rooms. Try it, its free. Works for me.

If you had to choose one, which would it be:

1. Place speakers away from back wall
2. Place speakers away from side walls

As you put it, 1) is the best choice. It is easier and less of an eyesore to control some of the side walls' early reflections so that they don't mess up yr sound too much...

Depends on the room though. In my room the best sound was with the speakers about 6 inches from the side wall but about 3 feet or more from the front wall. If I moved them back they sound terrible and if I moved them in the room from the sides they still sound good but not as refined. And close to the wall gave them a better soundstage because they were out of the line of sight more (physically removed from the soundstage).

The modes of the room will play a part in this though. When they were away from the side walls the modes would just totally come together more and drown out the soundstage and imaging. Also I just through some diffusers on the side wall and it was all good. Broke up the sound before it reflected off and ruined the soundstage. But for the most part those side walls can be very nasty.

I guess what Im saying is.... yes in general sidewalls are bad. But you have to just try it out and see. (I have them toed in pretty good too which will help a lot to make those side walls a lot less of an issue)

Front Wall.

The convention is that the front wall is the wall in front of you when you are sitting in your listening chair (the wall behind the speakers).

Another way is to think of the speakers as at the "front of the hall" as in a theater. The 'rear' wall is the one behind all the seats ;--)
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By the way, which is correct...front wall or ball wall when referring to the wall directly behind the speakers? I have seen it referred to both ways...just to be clear, in this instance I am referring to the wall behind the speakers.

Side wall first reflection will effect higher frequency more so than the front wall. Tweeter directionality will also limit front wall interaction unless it is a rear firing design. Side and front walls will effect lower frequency equally.

Some speakers; particularly UK, are designed for near front wall placement due to limited space in many flats.

Side walls