Quote from DIYA, and similar to what I have found:
"15mm BB is on the same order of stiffness as 1" MDF, and much less massive. So to bring the resonant structure of the MDF in line with the BB you'd have to add on the order of 2x as much bracing to the MDF. The MDF would still have much greater energy storage & retransmission."
In summary, BB sounds better.