Personally, if I were in a position to have a conference AND a prep, I would rather have an extra period for band. While having 2 off periods sounds nice, I imagine having larger classes, more scheduling issues, and less homogenous settings would not be worth it for me. Especially for the future (say you need to add a 3rd band or a jazz band or something. Would you want to have 4 beginner classes or just 3?)
For the record, I teach two bands and have five beginner classes in an eight period day with 1 conference period and no prep.