You do want the mech media to promote and give an area for beneficial bacteria to grow.
Although Matix has better reviews as to bio-available surface on a whole,the flat surfaces were not conducive to allowing bacteria to spawn over the entire media's surface/pores. IMO
two flat surfaces contacting each other cut that bio-available space off completely.
I decided to go with the Eheim Substrat Pro (AC50, Fusion4, and other filters being changed over) since the round surface guarantees that (nearly) the entire surface was available to promote bactarial growth around and inside the media. My future Fluval 305 will have it as well.