Front or Back wall diffusers

That's really a loaded question. There are endless variations, opinions, rules and suggestions to treating a room. I used all the above when treating my room but ultimately it came down to what sounded good to my ears and not so much following the rules to the letter. Start small and focus on treating your room to improve sound quality, not to look cool. 

Just load up the room with a couple dozen 5’ - 6’ artificial Ficus trees. They are cheap and very effective. Use along the walls, between the speakers and alongside and behind the listening position.

Ficus! Yeah...or go with furniture like I do...I like my "room sound" and no, you can't come over and hear it.

You have a very nice system.  If you diffuse the front wall between the speakers they will image even better which is really saying something.  The idea of diffusion is to scatter the signal so the reflected sound doesn't return to the listener as a single competing signal.  You don't want a second signal competing with the direct signal from the mains.  As a rule of thumb one cannot over diffuse a room so you can be aggressive with the amount of area the diffuser covers on the front wall.  If you diffuse the front wall and use bass traps in the corners the DAWs will really sing.  Good luck and Cheers

This is my back wall, I have absorption panels just below my on wall slim subwoofer. To the right you can see the Auralex Pyramid Sustain diffusors which are back filled with polyfill so they double as bass traps. It doesn’t have to be either or. In this pic you can’t see the treatments to the left of the sub but it alternates, absorbers to the left of the sub, then diffusors, then absorbers, etc.