I think ceramic media has the edge over bioballs, but it would be a close race. Ceramics probably "sit still" a little better because of their weight, which I would conjecture might be better for not disturbing biofilm structures and of course ceramic rings have the added feature of "randomizing" the small streams of flowing water, which is what you want in the media area. Ceramics might also have the edge in length of useful life. I've also heard some talk that very, very coarse sponge material (almost plastic brillo pad type stuff) may be a very competitive biomedia. Overall though, either of these in your question is considered a very good biomedia I believe.
~~waterdrop~~