In reply to broccoligirl: silk is warmer apparently, lighter and feels so slinky getting in to it.
i have a Cacoon cotton one i have used for years, used so much i have washed it back to white from cream! but it is brill, if it is hot i just use the liner, saves the bag wear too!
personaly i would go for cotton, but as i say it is personal preferance.
Jess.x