it depends what you use them for really
the BB`s are much faster and are best used as a screen in an attacking army IMO (altho byz cav arguably are more useful here) while trebs i feel are better suited to a defensive role
generally tho its a lot easier to get a hold of the trebs and they form the backbone of my byz armies along with the byz inf
i rarely build BB`s build do occasionally hire them as mercs when i go on the offensive