I voted for the black belt but honestly, I'd grow them up a bit and just keep the best looking specimen...both are beautiful fish and I'd tend to prefer black belt over bifas but you never know, you may wind up growin a more colorful than usual bifasc.
Both are amazing fishes, I just voted for Bif because I have never kept any before. I used to have a 12"+ male BB a number of years back and he used to push Midas and RDs his size around.