Which factor would have a greater impact on reducing GM, assuming equal dimensions?

Having a tank flooded near the upper deck would have a greater impact on reducing the metacentric height (GM) because it directly affects the center of gravity of the vessel. The stability of a vessel is influenced by the position of its center of gravity concerning the metacenter. When a tank is flooded near the upper deck, it raises the overall center of gravity as the added weight of the water shifts higher in the hull, decreasing GM.

When the tank is partially filled above the waterline, it does not significantly contribute to the center of gravity being raised because the water's weight is not as concentrated in the upper parts of the vessel as in the case of a tank flooded near the upper deck.

If the tank is flooded below the upper deck, the effect is somewhat mitigated compared to flooding near the upper deck—the water weight contributes to a lower center of gravity, thus minimizing its adverse impacts on GM more than flooding higher up would.

An empty tank would not affect the center of gravity or GM because there is no additional weight of water to raise the center of gravity, hence it has the least impact on stability.

Flooding tanks near the upper deck positions weight higher in the vessel, significantly reducing GM and affecting overall stability.