I have tried both and prefer diffusors at the first reflection point. Frankly I don’t understand why so many people recommend absorbers there because each time I have tried that there are some strange tonal balance changes, the soundstage becomes smaller and the highs are too dead sounding.
I've got some room issues myself. I do need some diffusion behind the stats (I have some junk piled behind them now) and already have some picked out. The stats don't project out to the sides, so I don't have to worry so much about sidewall reflections so much.
But I'm getting a reflection from the rear wall, and I'm debating absorption vs. deflection. The reflection is such that it sounds like I have rear speakers playing. I wouldn't deaden the entire rear wall, but there is one empty spot that reflects where I want something absorptive-ish.
I have diffusers at 1st reflection and on the center of the wall behind speakers.
The addition to the front wall was a very nice soundstage improvement.
I sit pretty much right up to the back wall and found absorption best there.
With the stats radiating front and rear, I've been warned about using absorption. But something that is both diffusing and absorbing works well, which is why I've already chosen some panels from GIK Acoustics which will achieve that for me. The room is kind of tight, so I don't have the luxury of a lot of space to work with.
They now offer an absorption panel that you can have printed with a photo of your choosing. The one inch panel wouldn't absorb everything, which might be preferable for my situation. There's enough remaining wall back there that things won't be completely dead. In fact, I've sometimes laid a 14"x14" pillow on the wall and it has helped.
I've messed with this a fair bit over the years, and what I liked best tends to be room and speaker specific.
When I used DQ-10s, I liked diffusion behind them and some space between them and the back wall, first reflection sounded best with absorption on the side walls and floor, diffusion in the ceiling. In small rooms were I sat very close to or even at the rear wall, I always preferred absorption behind me.
My current room has a favourable HxWxL ratio, is flexible as to speaker and listening positions, I'm using speakers that are point source and image very well, even near-field (Altec 604s), and I have no real limits as to how goofy I get with room treatments as it's a dedicated listening space.
After lots of experimenting, I like it best with main speakers out a bit from the wall and toed in a bit, I sit well away from the back wall and have absorption on first reflection point sides (heavy acoustic panels), floor (heavy carpet with under-pad) and a ceiling, that is a mix of reflection and absorption (plywood panels with cut-outs to expose open stud-work and some fibreglass batt absorption). I use a little absorption behind the speakers and almost a full wall of diffusion behind the listening position ('skatter' to some as it's not engineered reflection structures).
I have learned that is is easy to over-do absorption too, as it is a lot easier to do than controlled reflection - a room too 'dead' is easy, balance with less of that high-frequency absorption is more of a challenge. I tried to get there by keeping the reverb time fairly even across frequencies by keeping absorption somewhat limited, tune-able (if possible) and focused where it contributes the most, which in this narrow room (12'4"), is the side walls. With these speakers, bare walls or diffusion at the side walls lead to a kind of smearing or 'ghosting' with the image, absorption locked things in with better focus, extension beyond the walls (and weird effect!) and even depth. Another speaker dependant benefit in this case was that first-reflection absorption seems to help tame the 604 horn midrange shout a bit - something the right crossovers essentially do, but this is a little icing on the cake.
I have several of the absorption panels I use in varying sizes, and they just sit on the floor so are easy to move or remove from the room, so use this to 'tune' in the reverb times to adjust the apparent space to a certain extent. I've settled in to a tune that leans towards small and intimate (listening to a lot of Jazz, Blues, Folk/Americana lately) but it's fun to yank a bunch of panels to crank it up for rock or large-scale classical every once in a while. Even then though, the last panels I would pull out in this room with these speakers are the first reflection point panels.