Read you manual
Quote direct from page 13 of the manual:
1. Filtration always begins with a mechanical stage of waste particle removal as water is drawn through the foam that lines the perimeter of each basket. It continues as water is pumped "down" through the "media" in the three baskets.
2. Bottom media Basket, This basket receives the purest water stream, making it the proper place for dedicated biological media.
As for Eheim having a better bio efficiency, this would depend on the media used, as the FX5 doesn't come with it's own media i would like to know what they used as a substitute. Although the filter states the pump has a flow rate of 3500l/h which is true for the pump, page 4 of the manual also states that the filters circulation is only 2,130l/h so the fact they could prove the output wrong was never that at all they simply didn't read there manual.