diffuser or absorption panels, which would you choose?

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.  

Both I have bass busters in the corner then acoustic foam panels behind the speakers 2x4 panels as well as behind above, and first reflection panels on the side and a carpeted floor , it makes a Big difference no slap echo and still lively sounding.

The answer would depend entirely on a measured room response taking into account the on-axis response of the speaker. If the speaker has a flat on-axis response and room response mids-highs are elevated, I would pick absorption. If the room response is good, I would go for diffusion.

I would also consider the music I am listening to. A lot of "near live" stereo microphone recorded music and I would want broadband absorption because I cannot get a wide enough frequency range with diffusion. If I am listening mainly to studio, I would not be as concerned.

I see others have gone beyond the simple request in their answers. There is no single solution. Front wall treatment varies depending on what speaker to wall distance you are targeting. Room dimensions come into play, angles between speakers, walls and listening position, and as noted, your starting position concerning room response. Also, whether you will equalize / use DSP.

Sounds as simple as a football bat.