Actually, the only reason this is close is optics. Bird actually was a pretty good athlete, but he couldn't jump, and he was below average laterally. It just looked harder for Bird.
Pierce was a much more fluid and explosive athlete, with an almost perfect balance of every important offensive skill. He missed, but he never looked like he couldn't get off his shot.
But if you then consider the numbers and the actual performances, while Pierce posted numbers that were worthy of a major star, the numbers say Bird was substantially more efficient. He was the founding member of the 50-40-90 club.
Either guy could go out in a big game and beat you practically by himself. But Bird did it more often, and he did it with less shots.