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.