Hi there, I don't know a lot but I can tell you some of the basics.

Fleece covers - i still use them at the moment and have no problems. But fleece is a material that wicks - so when it is wet it disposes of it's wetness to the outer layer of clothing.

Wool coveres - Wool can absorb up to four times it's own weight before it is 'wet'. It is also a breathable material and can dry during the night, then of course absorb more, thus making it more absorbant. I think you can get compression leaks from wool but I'm not 100% on that.

Hopefully someone will pop in soon and give you some better info!