Agreed, you want a surface that allows bacteria to grow on it but disallow the collection of debris. Sponges will fill up with with debri and will likely impede the flow of water - which is critical to keeping the bacteria alive and well. Eheim bio balls are second to none really.
Unless, of course, you read this article. I've used Matrix for years. It's much less expensive than Ehfiwhatever and I've had no issues with clogging. Of course it's probably 6 of one and a half dozen of the other when it gets down to it.