This is a complex system type question, so there is no easy answer. To aid you, let’s look at the logic and the fundamental principles, hopefully without subjectivity of the materials practicality. The method of cotton clothing is essentially a fruiting cotton field, a harvest and cleaning process, the looming process, then you have something you can sew into garments. Synthetics, essentially, involve shipping in some sort of petroleum byproduct from a god forsaken place, and chemical engineering it into small molecule polymers, then you have something you can work into pants, jackets, and the like. Each product is quite different in manufacture and performance. Ever heard of apples to oranges?
Anyway… so, if “environmental” means “sustainable and logical” then which is better? I’d assume the field-and-plant-based approach. However, there are always subtle nuances to these complex systems. Such as how land is bought and managed, market prices, consumer demand and the supply, logistics, weather, death, and taxes; as well as other fruits.
In the end though, I can’t rightly say, as I have no data. I can speculate though. Which is my favorite thing by the way. And I speculate that cotton fibre may be a bit better for the environment than synthetics. The reason is that petroleum products are nasty and cotton is lovely.
Yet, there is a thing called valuation, where you determine factors that are worth valuing. And you can focus this within an environmental and social paradigm. The way it works is that you determine something to value and find the value by asking folks that care about it how much they value it. Then, you find out how much money it could be bought for by some fatcat. Then you think how long it will last into the future. And on you go finding things about it that you value. So, if you apply this idea here you would look at the many factors involved with each clothing process as it relates to what you have determined “environmental” means. Then you apply subjective methods of valuation using as much quantitative data as you can get but being willing to give in to emotion or social desirability and overvalue something due to it’s popularity. That’s value baby!