For starters, cotton and polyester can’t be compared apples to apples because the base fibers have a completely different structure. They both have pros and cons. Sweaters happens to be a product I’ve specialized training in so now you get my brain dump.

The fiber staple length, number of yarn ends, the twist of the yarn, the yarn size, the stitch, the gauge of the stitch, yarn dying method and yarn finishing treatments all factor into the longevity of the garment. However, 80% of the cost of a sweater is determined how long it takes to knit, followed by cost of yarn, and finally trims and seam finishing method. It’s literally impossible to do an apples to apples comparison of the same garment from 3 different factories (trust me, we’ve tried), let alone different garments from different retailers.

Mercerized cotton vs short staple, single twist cotton is going to perform totally differently. The former will hold it’s color, keep it’s shape and rarely pill but it’s going to cost you 4x as much as the later. Carded cotton is somewhere in the middle because they pull out the longer fibers, which pill less and has higher tensile strength. The cheaper the garment, the lower the quality of the fiber, and the faster the garment will wear out.

Polyester is man made so any number of different features can be engineered into the fiber. Rarely is a sweater 100% polyester, so the other fibers in the yarn will effect it’s performance. Acrylic, cotton, wool (sheep or alpaca), nylon and spandex are common blends, and each of these has its pros and cons. Generally, poly will retain it’s color, has high tensile strength and usually doesn’t pill due to being one long continuous filament, but it’s not soft, and doesn’t breath well. Acrylic is soft but it’s a pilling nightmare, as is most wool (mercerized wool is the exception). Nylon and spandex are usually added for stretch and will break down in the wash over time.

TLDR: sweaters are complicated ;)