How often to aircraft catch fire enroute, where the flame makes it into the cabin and injures the occupents, but the fuel tanks don't catch fire, so the crew has enough time to land and evacuate? Any other senario, I can't see the clothing you are wearing making any difference.
In a ditching situation, cotton would surely be worse, as it would hold the moisture, and ensure you lost heat quicker from evaporation.
I'm not saying cotton wouldn't be better in a fire situation, but rather that the risk is so small as to be irrelevant, and no more serious that in a car accident. In a car accident, you are much more likely to be unable to exit the vehicle, at least in an aircaft, you stand some chance of being able to make a normal landing.
dp