Definitely the front wall, in my opinion.
I've posted a photo of the front of my room in the Virtual System area. I've had three pairs of GIK Gotham Quadratic Diffusers deployed in the middle of the room boundary behind the speakers for over a year (I had a single pair before that) and have found they make a substantial difference—the perceived size of the room is larger and the sound is smoother. Because I'm a reviewer, lots of speakers come and go, and I've found the GIKs to be effective with various loudspeaker types—dynamic and electrostatic designs, ported and sealed box, floor standers and stand mounts. As room treatments go, they aren't expensive ($359 per pair) and even my wife doesn't find them unattractive.
In the photo, they are mounted in heavy wooden frames I had built. The speakers in the photo are JansZen Valentina P8s, which I own especially for use with the BACCH-SP crosstalk cancellation processor but the GIKs also are of considerable benefit when I'm listening to my Magicos.
Andy Quint