Front or Back wall diffusers

That's a generous sized room. The main thing to remember is you are treating a system that consists of the speakers, where they are in the room, and thus how the acoustically load the room, and the room characteristics themselves, including all walls ceiling and floor. No one wall or surface is more important than any other except for two things: 1) Controlling first reflections on the side walls and ceiling. 2) Bass traps are different, as corner placement is far more effective.

Acoustics isn't opinion, it's applied math and physics,  and it's pretty well documented and computer modeled at this point. A room of your size will require somewhere between 100 and 140 ft2 of 2" acoustical panels between the 4 walls and ceiling, plus 2-4 Bass traps. Figure $12-15 per ft2, installed for 2'X4' panel or around so maybe $2500 all in, depending on your choices of fabric and local labor rates. The panels aren't hard to DIY install, but it's definitely better with 2 people and appropriate ladders. A pro will also use a laser to make sure panels are installed square and level.

Take your time, do the math and enjoy the results. You'll wonder why you waited so long. 

A room of your size will require somewhere between 100 and 140 ft2 of 2" acoustical panels between the 4 walls and ceiling, plus 2-4 Bass traps.

Without knowing more about OP's room -- wall materials, rugs, furniture (especially couches, etc.), windows, etc. I'm not sure where in this estimate his room falls, indeed if it falls within this range at all.

No disagreement that doing the math is important, but the math can only be done if as much as possible about room contents and materials is known, first.

GIK and companies like that need to sell product. Yes they analyze your room but they are going to try to sell you as much as they can too. Most of the people on 

Oops! Most of the people on this forum have the means to consult a professional acoustician and I am surprised most don’t. Jeff at HDacoustics is very good and will analyze your room and tell you what is needed and where to put it. He has vendors he works with as far as panels go but he doesn’t push you to purchase. You can use his design and go from there if you want. Good luck! 

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.