diffuser or absorption panels, which would you choose?

Check around your area to see if you can find a local builder. I found a guy via FaceBook (which I never use for such things) and he built panels and bass traps for a fraction of the cost of some of the ones you'll see offered. Granted, there's nothing wrong with those services, room correction, etc. Please don't hear me dismiss that. But I was able to build, add, and add a few more, finally with 2 diffusers for several $100 vs. thousands. And custom made, same insulation options, and I was able to get the precise sizes for my room. 

There is no such thing, you need both. Any research you do will tell you that. Wish it was that simple.

OP

Diffusers behind the speakers on the front wall. Absorbers on the front wall will close the stage, speakers will not disappear as the stage will be behind the speakers; don’t believe, try it. Diffusers on the front wall will disperse the reflections and you will hear an improvement in both depth and width, and speakers disappear, at least that’s my experience. Putting a few random panels up on the front wall or only behind the speakers will not help much; best is as a large panel between the front speakers.

Multi diffusers/absorbers best on the side wall at the first reflection. You don’t want to kill the room but you do want to control it so the sound from your speakers will  hit your ears before the first reflection of the sidewalls. I do nothing on the back wall as you want reflections from the room. Speakers need a room for many reasons… it’s about controlling the reflections. Simple test, is take your system outside and hear how horrible it will sound. 

Can’t just do one or the other. Vicoustic is another manufacturer that make both products, which is what I use. Every room is different, and everyone has their opinion what’s right, so buy both types of panels and return what doesn’t work for you.

I suggest watching the hour long video because a lot to learn, and maybe watch others as you’ll probably discover they all support it each as it’s science/physics and can be tested and proven. Lastly pull your speakers into the room and same with your listening seat off the back or unlikely any treatment will help much.

Have fun and good luck.

Really depends. Assuming the room is mostly reflective, absorption.

You can kind of, poorly, simulate diffusion with absorption by using relatively small abosrbers (2’ x 2’ or smaller) and scattering them instead of making large solid areas.

So, I'd chose absorption over diffusion for this sake.

My experience matches what others have said.

Too much absorption will make the music sound dead, but some absorption is likely necessary. I tried the GIK Diffusor/Absorbers behind my speakers to start with and found the sound to be kind of flat. Replaced them with QRD diffusors and this made a big improvement. Deep, wide sound stage with a lively sound. 

Absorption or a combination of absorption and diffusion for the side walls (and ceiling), particularly at first reflection points, also helps a lot. I like to use "art panels" like those offered by GIK and others, with scatter plates which provide a mix of absorption and diffusion, and can be fairly attractive. 

If you have a small room and have to place your listening position close to the rear wall, absorption with a bit of diffusion (such as the GIK panels mentioned above) can make a big difference to the clarity of the music since your ear/brain has a hard time distinguishing the reflections from the direct sound when they are spaced so closely in time. 

Don't let anyone tell you that some rooms can get away without acoustic treatments, at least if you are striving for the best sound. In my experience, targeted acoustic treatments offer the best sound quality improvements for the money spent than anything else I've done to my system.