Ceiling vents are more efficient for air conditioning output and return vents than floor vents. However, the opposite is true for heating systems, for which floor vents are a more efficient choice.
The most efficient air conditioning and heating designs take advantage of natural air flow patterns as much as possible. In the case of cooling systems, ceiling exhaust vents allow for cooled air to naturally circulate to the bottom of the room, while warm air rises to the return vents. For heating systems, floor vents allow heated air to circulate easily through the room and capture cold air as it sinks to the floor.