Plants can metabilize ammonia and urea, so for the plan't sake, it doesn't matter. The best of both worlds is a gravel bed F/D feeding a FT for stuff like lettuse. The gravel bed IS the biofilter. My system uses an aquacube in with the fish for bio-conversion. That way a pump failure, or excessive harvesting doesn't upset the apple cart bio-wise. The air and pump are on separate circuits just in case.