I cant say for certain, but I can definitely speculate. I do know cotton requires a fair amount of water to grow, but I don't think it would use as many petrochemicals in the production. Though it would still use some, even if that is just in the supply chain through things like diesel for trucks and ships. The chemicals they use (like pesticides) may be derived from petrochemicals, but even if they aren't they could be damaging to the environment in many other ways. So I think polyester could have the greatest emissions of the two.

I guess it could depend on the scale of production too. Like if we were to try and replace all polyester clothing with cotton, that could have a massive impact due to the amount of land and water needed to produce such quantities of cotton clothing and such. But at the same time, creating clothes out of plastic isn't going so well either.

Ultimately we will probably still have to have some diversity materials for sustainable clothing production. It will really come down to a balance of land use, water use, what uses the least amount of chemicals, and probably a lot of other considerations.