Absorption, Diffusion or a combination of it all?

Diffusion properly placed adds fullness and depth to the imaging.  Usually listeners like diffusion directly between the speakers and directly to the sides, and behind the listening location.

A 100% absorptive room sounds terrible. :)

All of it ! alternate absorption and diffusion , use absorption at the first reflection points.

From what I’ve read, which may or may not be accurate in all instances, is that you want absorption behind the speakers (front wall), and diffusion behind the listening chair (back wall).  Based on experience, I can vouch that the aforementioned holds true for electrostatic (dipole) speakers, but I'm not sure that it carries over so definitively for regular speakers. 

OP. Anthony Grimaldi has an "acoustic recipe" based on thousands of rooms he has tuned. You may want to tweak it a bit but it gets you extremely pleasing results as a starting point. It starts at the :49 min mark in this video:

https://www.youtube.com/live/G0ekssXX7rE?feature=share

Both, is typically the answer.

Diffusion - and even deflection - has been the greatest help for my system, once the bass was tamed (with absorption).