Hi Parisgirl,
That’s one of the benefits of a three layer component system with more options for customization…and can also be one of the downfalls for others.
Controlling humidity is extremely important for maintaining a comfortable sleep environment. Both cotton and wool are good at absorbing moisture, with wool being better at more efficiently releasing it back to the environment. The cover on the 8" special is a cotton stretch-knit and it is thinned and would be a bit more breathable than the cotton/wool cover, but both would be good choices, and are only one part of controlling the microclimate of your mattress.
In very general terms … the materials, layers, and components of a sleeping system that are closer to your skin will have a bigger effect on airflow, moisture transport, and temperature regulation than materials, layers, and components that are further away from your skin and softer mattresses or foam toppers will tend to be more “insulating” and for some people can sleep warmer than firmer versions of the same material. The type of mattress protector and the sheets and bedding that you use (which in many cases can have just as significant an effect on sleeping temperature as the type of foam in a mattress) and on where you are in the “oven to iceberg” range and because there is no standardized testing for temperature regulation with different combinations of materials. There is more about the many variables that can affect the sleeping temperature of a mattress or sleeping system in post #2 here that can help you choose the types of materials and components that are most likely to keep you in a comfortable temperature range.
Phoenix