You are correct that the volume of air displaced by two objects with different densities will be different, and that this could in principle affect their weight when they are placed on a scale. However, the effect is negligible in most practical situations, such as measuring the weight of objects on a laboratory scale.

In a laboratory setting, the effect of air buoyancy on the weight of objects is typically corrected for by using a scale that measures the force required to balance the object against the force of gravity. This is known as an "analytical balance" or a "top-loading balance." The air buoyancy effect is taken into account by measuring the weight of the object in air and then subtracting the buoyancy force.

So to answer your question, in a laboratory setting, a kilogram of iron and a kilogram of cotton would be considered to have the same weight, as they both have a mass of one kilogram, and their weight would be corrected for the effect of air buoyancy. In everyday life, the effect of air buoyancy is often negligible, and so for most purposes, we can consider a kilogram of iron and a kilogram of cotton to have the same weight.
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