Comparisons between natural and synthetic fibres usually settle on a simple verdict. The underlying figures do not support one, because the two fibres carry their costs at different points in a garment's life.
Cotton's burden is agricultural
Cotton is a crop, so it occupies land, requires water, and in most growing regions involves fertiliser and pest management.
Water use varies enormously by region and by whether the crop is rain-fed or irrigated, which is why single figures quoted for cotton are close to meaningless without a location attached.
Land use carries an opportunity cost as well, since the same ground could grow food or remain uncultivated, and that cost does not appear in any price.
Polyester's burden is upstream and downstream
Polyester is derived from fossil feedstock, and its production is energy-intensive, though it requires no farmland and very little water.
At the other end it does not biodegrade in any meaningful timeframe, so a discarded polyester garment persists.
Washing releases microfibres that pass through treatment systems, and this shedding occurs throughout the garment's life rather than only at disposal.
The use phase often outweighs production
For frequently washed items, the energy used in laundering and drying across a garment's life can exceed the energy used to make it.
Polyester dries faster and needs less heat, which reduces that ongoing cost. Cotton absorbs more water and takes longer to dry.
Durability enters here too, since a garment worn twice as long halves its production impact per wear regardless of what it is made from.
Recycled versions shift the problem rather than removing it
Recycled polyester mostly comes from bottles, which diverts material from one waste stream into a form that is harder to recycle again.
Recycled cotton is limited by fibre shortening, so it is blended with virgin fibre and cannot close the loop on its own.
Both are improvements on virgin material and neither eliminates the underlying trade-off, which is why claims resting solely on recycled content are incomplete.
The comparison depends entirely on the garment
A rain shell benefits from synthetic performance that cotton cannot provide, and using cotton there would mean a garment that fails at its job.
A shirt worn against the skin and washed constantly makes a different case, where comfort and laundering behaviour matter more than tensile performance.
Choosing the fibre that suits the garment's actual use, and then keeping it in service for years, does more than substituting one fibre for the other across the board.