I am sure someone no here can offer up a nice link for you as to explain more indepth the ups and downs of filter sponges. What you will find though if you run this like an experiment, is that with a sponge filter in your system you will have a tough time getting your nitrates down to zero. Sponge filters are often referred to as "nitrate factories". With proper live rock and a good skimmer there should be NO need for any other filter media. It be as simple as that old saying "keep it simple stupid", or you can go scientific and say something like "all there is nature is rock and sand and natural protein skimmer, there are no bio balls or sponges in the ocean".
But I absolutly believe in research, and learn from experience. So keep an eye on your nitrates and then pull that filter out, you shouldn't see anything negative from removing it, and you nitrates should drop without it.
I think you would be hard pressed to find an experienced reef keeper who uses mechanical media...we will see what some of the others here have to say.