I prefer cotton leggings to poly, but that’s also because I work with fire a lot and polyester is dangerous around fire.

But aside from that specific case, cotton is so much comfier than synthetics, doesn’t trap heat, and lets your bits breathe. Also fewer microplastics. It’s t-shirt material.

(I agree with the other person though, for the love of FSM please stop asking about every single little detail related to leggings)