retrolefty:
Well I'm very surprised at you, don't two ping pong balls have the same wind resistance even if one is hollow and the other filled with water? Read the actual experiment the OP is proposing.
The aerodynamic drag would be the same, but the impact of this on the acceleration would differ. If the ball is very light then the drag would be very significant relative to the weight and the ball would reach terminal velocity at quite low speed. If the ball was filled with water the drag would be the same (at a given speed) but would be less significant relative to the weight. Just imagine the two balls dropped from an infinite height and imagine the speed/time curve as they each accelerated up to their respective terminal velocity. The empty ping pong would end up going very slowly, and the filled one would be going much faster.
The classic 'cannon ball' experiment only works when air resistance is negligible. It is negligible for iron balls dropped a few tens of feet, but it's not negligible for ping pong balls dropped a similar distance.