Turtle, I've asked that same question before. If you believe the manufacturers, the biomax should be much better, but they have a tendancy to exaggerate things...
In either case, I believe the biomax would be better. It is possible to get a lot of surface area out of something if it is designed for it. So, No, I don't thing an extra sponge would give you as much bio capacity as biomax would. In my case, I have biomax and two sponges. A single sponge and two bags of bio would most likely be better, but I'm already well over what I need, so it would be pointless gross overkill