What is the difference between poly poplin fabric and cotton poplin fabric?
The main difference is that one is made of 'poly' (likely polyester) and the other is made of cotton.
How notable the difference is depend on how the poplin is made and how they handled the threads from which they made the fabric and the fabric once made.
Mostly man made fibers are more smooth and shiny but it is not a given.
I own a bag made from recycled pet bottles and when you see it you would say it is rough cotton. I own fine cotton fabric which is shiny and smooth like polyester in the way it looks.
Cotton mostly feels better to the skin, not as likely to make you skin sweat.
So best go with the descriptions and pictures on the site.