I have both, cotton for s specific 'nice' dresses' and some lightweight polycotton that I've seen and thought better than the average to have in the stash for quick unplanned makes, especially little summer sun-dresses which may only get worn a handful of times. I always have some white cotton and polycotton on hand to use for a contrast band or yoke etc to make another outfit from the offcuts. I love it when I get a bonus dress or top that way.