As c2 expierance is with wet/dry filters for aquariums her design makes sense. If you ever visit a saltwater/freshwater aquarium forum they will preach to you that the bio-material should be at least 2/3's above the water line and preferably all above the water line. The trick is to have a good dispersion over all the medium, and its here C2, your design needs a little work.......might a suggest adding spraybars/drip tubes to your T's for better distrubution over the medium?
However, that being said, the bottom up method provides an infinatly better filtration of solids, and C2, eventually your bio-balls will become filled with sediment/sludge and become a toxic waste machine without some type of pre-filtration (does your skimmer have filtation?). Not to mention you will have ever insect in the neighborhood atracted to it as bio material accumulates in the top layer of bio balls. We don't have this problem in enclosed houses where we use this type of design.
That being said I guess the best of both worlds would incorporate both , perhaps with a 2 barrel system in which the first barrel flows from the bottom up, like a skippy design simular to Koiguys, and then the flow out the top of that barrel would then be directed to a second barrel that flowed from the top down like C2's design.