wool vs pul

ok so am pretty new to this cloth nappy world (about 2-3 weeks of using them so far) and i am getting there but do have a few questions about covers..
the two nappies we are using that require covers are terry flats and baby beehinds fitteds.. we use the flats during the day at home and the BBH at night.. the covers we have are limited so far we have two what i think are fluffies?? they look knitted on the inside and well fluffy on the outside?? someone gave them to me so i am not sure exactly what they are.. a few of those cutie ones from big W which are great but are now getting covered in lint and pilling badly each wash and one BBH PUL cover which i really like.. ok so get to the point noni!!
my point is i am obviously in need of some new covers i love the BBH but am convused about this bussiness with wool?? why do people prefer a wool cover at night? is it just because it seems to go the distance better so to speak or is there something else i dont know??

The fluffies covers are polyester. Kinda like a fleece cover.

Wool is better for nights because it breathes so much and gets airflow so that the nappy actually dries a little bit through the night with the airflow, giving more absorbancy. Also wool can absorb up to 4 times its weight in liquid before it feels damp (which is why you need to dry them for longer than you think), whereas a PUL cover will just get wet and start wicking onto the clothes. Wool not so good with compression though, so if you have a soaker under a tight fitting bodysuit you might find the clothes get wet.