Wet/dry bioballs have some that will be dry and some that will be wet. The problem with the dry side is the nitrates accumulate on the dry bio balls, thus causing them to get trapped, which can result in elevated nitrates. With the reef rubble, it will be fully submerged eliminating this issue. Hopefully I explained this in a sensible manner.
Some people still use bioballs and swear by them. It's all a preference. The Berlin filtration is ideal, and more natural. HTH