Question

There are two thing 1 is iron of 1kg and another is cotton which is also 1kg which one is heavier

Answer 1

No one is heavier both are equal as you have told it is 1kg

Answer 2

Both are equal
That means it will weigh slightly less because there is less gravity acting on it (well, I suppose it's the same amount of gravity, just some of it is pulling it away from the centre of the earth). But obviously if you have 1kg of cotton and 1kg of iron on the earth's surface, they will weigh the same.