I am looking to buy this maternity dress online and there are 2 types of materials. One is 95% polyester and 5% spandex and one is 95% rayon and 5% spandex.

Can anyone please explain to me the difference? It's one of those dresses that wraps 20 different ways and I am buying black so want to be able to dress up and dress it down and have it to wash and wear.

Thanks in advance