It is hard to state which is best not knowing what the overbank looks like. If you think the area with the buildings is going to be just storage and no velocity in floodplain then ineffective would be a good choice. If there are a lot of buildings, enough that you would have difficulty capturing their effect, and you feel there would be velocity in the floodplain I would go with a higher n-values to reflect impact. Now if just a few buildings I would use blocked obstruction. The blocked obstruction will add wetted perimeter for the walls and potentially be more accurate.