I have done a number of QAYG - I always used cotton batting - the last QAYQ I used a poly because I wanted it to be fluffy - I had never used poly before and it was a disaster for me - not one square came out the same size - I am not sure why this happened -I was doing a complicated pattern "Snail Trail" but had an excellent teacher. you have to be very precise in your measurements and drawn lines if using poly. Other people might have a better explanation and what to look out for when using poly versus cotton.