the "pros" for poly are price, it doesn't shrink, and it's easier to use when hand quilting. the "con" is that it's much more slippery than cotton, so i've always found i needed to pin and/or stitch-baste a lot more if i don't want puckers on the front and back when machine quilting.
the "pros" for cotton are that it grabs the front and back fabrics more, so it's easier to baste and machine quilt with fewer puckers. (someday i'll be able to say "no puckers". :lol: ) also, if you're looking for a flat look after quilting, cotton is the way to go. the "cons" are price, price, price, and the fact that you don't always get what you pay for when you fork out that price.
i've never understood all the angst and panic over fumes and flammability. if a fire has reached the bed, you already have plenty to worry about and not much time to get the heck outta there, anyway. :?
everybody has a favorite brand, regardless of content. experiment with the different types and brands that fit into your budget. when you find your favorite, you won't want to use anything else no matter what anybody else says. :wink: