I ALWAYS put the bioedia after the mechanical filtration and here is my rationale for that.

If the biomedia is first, Every bit of particulate matter [or any matter at all] that your filter picks up gets slammed into your biomedia and immediately starts clogging the microscopic pores that the BB live in. This obviously lowers/limits the BB capacity of the media which ultimately undermines the efficiency of your filter.

It's counterproductive to do it that way IMO.

I always have the mechanical filtration [Sponges/floss] first to remove that particulate matter in an attempt to maintain the purity of my BB colonys house. Obviously it funks up over time but I see no point in putting it directly in the line of fire - So to speak.

I haven't seen a more logical rationale for doing it the other way round yet, YMMV.