Is it better to buy secondhand synthetics or new organics?

I need sweaters for the cold weather, as I've recently lost weight and the ones I have are now too big for me. Is it more sustainable to buy secondhand from Depop considering most listings are synthetic blends, or buy new and make sure everything is 100% cotton? I personally prefer wearing cotton and I know that synthetics release microplastics if not properly washed. Sustainable brands I've been browsing are Toad&Co and Mate the Label.