When and if Galileo dropped two balls of the same size but different masses from the top of the Leaning Tower of Pisa, air resistance was not really negligible. Which ball actually struck the ground first?

Both balls struck the ground at the same time

Explanation:

At the moment when both balls fall from the top of the tower, they will reach the ground at the same time, as speed is not a function of mass. All the kinematics equations deduced by Isaac Newton, in his studies show that the velocity of a body in free fall is not in function of its mass.

The only way a body falls faster than another is when one of these bodies has a geometry in which air influences its velocity of fall. For example, when comparing the fall of a steel ball with a feather. But in the same way in modern experiments in vacuum chambers with limited air presence, it has been shown that a ball of steel and a Feather Fall at the same speed. Thus, the mass of a body does not influence its rate of fall.