Just one note. Observe other riders on roads or MUPs where you are likely to see large numbers of both. In particular observe how they hold their arms while trying to go fast or into a strong wind. Drop bar riders who are hunched down usually pull their elbows in such that the shoulder, arm, elbow, and hand are all in a line tucked close to the body. Flat bar riders who are hunched down often have their elbows stuck out in the wind, arm segments positioned more laterally than longitudinally.

This would appear to be due to hand position. The elbow can be pulled in when the surface of the palm is vertical, facing inward, typical when gripping either the hood or the forward throw of the bar. When the palm is facing downward on the flat portion of the bar the wrist doesn't bend that way comfortably. (Try it.) A drop bar offers both positions. A flat bar does not unless it has forward extensions, but if the brake levers are mounted on the flats the rider can't keep his hands on them. Flat bars, at least those meant for mtbs, are also much wider, which makes pulling the elbows in more difficult.

The sections of drop bars not really necessary are the drops themselves! One can extend the hands to the hoods with elbows bent, a position perhaps even more aerodynamic than the drops. However cutting off the drops would save just 12 to 14" of aluminum bar, and sometimes the drop position is nice for other reasons than aerodynamics.

It boils down to what you are comfortable with.